Invitations (#1122)

* Invite existing user to a site

* Add invitation flow for non-existing users

* Accept and reject invitations

* Use invitation flow for existing users

* Locking mechanism for sites

* Authorization for site settings

* Show usage based on site ownership

* Add ability to remove members from a site

* Do not show settings link to viewer roles

* Ability to remove invitations

* Remove `Plausible.Sites.count_for/1`

* Fix tests

* Do not show the trial banner after the trial

* Correct trial emails

* Transfer ownership

* Send invitation email to existing user

* Add invitation email flows

* Add plug for role-based authorization

* Rename AuthorizeStatsPlug -> AuthorizeSiteAccess

* Add email flow for ownership transfer

* Fix URLs in emails

* Fix small copy issues

* Make 'People' its own section in site settings

* Notify user via email if their access has been removed

* Check site lock status when invitation is accepted

* Check lock status when user subscribes

* Make sure only admins and owners can create shared links

* Changelog

* Add LockSites to daily cron

* Clean invitations after 48 hours

* Add notices about expiry

* Add invitation expired page

* Add doc link