* Make membership creation and role updates more explicit in terms of changesets

* Extract invitation accept flow logic and refactor it slightly

* Improve acceptation logic

* Update moduledoc

* Improve SiteLocker API and add typespecs

* Stop naming function not returning a boolean like a predicate

* Refactor rest of invitation actions and safeguard against rogue requests

* Update code docs slightly

* Extend `Billing.check_needs_update/1` tests

* Parametrize selfhost flag and toggle SiteLocker logic on it

* Add tests for newly extracted services

* Add test case and a fix for locking site on grace period ended

* Make invitation controller tests async as there's no more env patching

* Add test cases for self-invites and fix one bug

* Add and refactor tests for rejecting and removing invitations

* Prevent issuing ownership transfer to existing owner

* Improve name of the test

* Improve `Billing.check_needs_to_upgrade/1` return value

* Improve `Billing.SiteLocker.update_sites_for/1` and its tests

* Fix typos

Co-authored-by: hq1 <hq@mtod.org>

* Make invitation removal and rejection resilient to races

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Co-authored-by: hq1 <hq@mtod.org>