Create a master repo for blueprints.tails.boum.org

It would be nice to be able to manage the ACLs ourselves but that shouldn’t be a blocker.

Our options:

host it at git.tails.b.o, if immerda’s setup allows us to deploy a hook that pushes to the repo used by ikiwiki at boum.org: we can manage ACLs ourselves, but it may not be possible, or not possible fast enough for us, depending on immerda folks’ availability
host it ourselves at puppet-git.tails.boum.org: we can manage ACLs ourselves, but that place wasn’t meant to host repos that many people can access
host it on boum.org just like the main repo: we can’t manage ACLs ourselves, but there’s no infra to set up and maintain on our side at all

Parent Task: #9174

Original created by @sajolida on 9178 (Redmine)

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