Extend headers to meta

This MR includes the Enc and Sig pseudoheaders in the list of configurable headers. This makes it possible to rearrange or disable them.
Fixes #297

See merge request schleuder/schleuder!147

Resolve "x-set-fingerprint sets empty fingerprint-value in subscription if there is an accidental line-break between E-Mail and Fingerprint"

Closes #360

See merge request schleuder/schleuder!209

Using the new introduced keyword an admin can now remove the linked
fingerprint from a subscription. A subscription can only remove
the fingerprint of themselves.

To unset their own fingerprint admins must additionally pass the
argument `force`.

Check for a valid fingerprint as argument and do not accept an empty
fingerprint. This will not anymore allow to unset a fingerprint
through the `SET-FINGERPRINT`. This functionality will be superseeded
by another keyword.

As part of that fix, wie centralize checking for a valid fingerpint
and constrain the check to be either 32 (v3) or 40 (v4) characters
long.

Add spec for data migration and remove inline class definition

See merge request muri/schleuder!1

Relates !207

Outgoing protected headers

Closes #74

See merge request schleuder/schleuder!205

CI: Debian: Add job to run autopkgtest; minor refactoring to keep things DRY; explain what lintian does; add TODO to introduce job to check package for reproducibility

See merge request schleuder/schleuder!206

Currently, the toolchain is still experimental: Using reprotest, even
with all variations disabled, makes the build fail.

Relates #324

Relates #324

Relates #324

Introduce mission statement

See merge request schleuder/schleuder!207

If the incoming message had a "protected headers" subject, pass this on
in a way that enigmail recognizes.

Result of mutual text work to improve the CoC

See merge request schleuder/schleuder!208

Result of mutual text work to improve the CoC.

See merge request schleuder/schleuder!203

* Since the AddSigEncToHeadersToMetaDefaults not only changes the schema
but also the data, we decided to have a test to ensure the bevaviour.
* The inline class definition that was used to reference the list class
with a temporary class that decouples the migration from the concrete
class implementation threw an rspec error. Therefore the inline
definition was replaced.

CI: Introduce jobs to build Debian package and run lintian on the result

Closes #324

See merge request schleuder/schleuder!202

Ignore spec folder when analysing test coverage

See merge request schleuder/schleuder!204

Resolve "matching keywords in body is too strict"

Closes #249

See merge request schleuder/schleuder!199

Lintian is a tool used within Debian, to check packages for errors and
compliance with the Debian policy.

Relates #324

Resolve "last/only remaining admin can self-unsubscribe via x-unsubscribe, leaving the list without any admin"

Closes #357

See merge request schleuder/schleuder!201

This changes the way how we parse for keywords to the following
approach:

  1. Ignore any empty/pure-whitespace lines at the beginning of
     the first plain_text_part message.
  2. If you match something /^X\-.*/ start matching for plugins,
     else abort.
  3. Once you matched a plugin, but you have a new
     non-empty/pure-whitespace line starting with something
     different than /^X\-.*/ abort looking for plugin names.

This fixes the most common problems with lines starting with `X-`
further down in the body of an email, while keeping all of the
existing behavior and functionality.

Any further improvements to the code base and the parsing of
mails as discussed in #249 is postponed to future work. Tis work
should go together with work in #359 which will allow to change
the structure of plugins.

Merge branch '354-admins-with-delivery_enabled-false-are-not-getting-results-of-mails-they-send-to-request-address' into 'master'

Resolve "Admins with delivery_enabled=false are not getting results of mails they send to -request-address"

Closes #354

See merge request schleuder/schleuder!200

This enables us to get fast(er) feedback in merge requests regarding the
Debian-side of things. It allows to spot errors and to find breaking
changes earlier.

Besides, this will reduce the time between the upstream release and the
upload to Debian. It enables to do necessary Debian packaging changes
while the new upstream version is still in development, in contrast to
postpone if after the actual release, which was the situation before
this commit.

Less pain, more joy to come. \m/

Relates #324

Replies to keyword-usage and notices to admins are now sent without
regard of the delivery-flag.

Handle more keywords without argument

See merge request schleuder/schleuder!196

Split API daemon

See merge request schleuder/schleuder!197

Resolve "Enable adding filters from external directories"

Closes #282

See merge request schleuder/schleuder!167