* Remove allowance_required field from grace_period

Since we are now preventing customers from subscribing to a plan that
does not accommodate their pageview usage, there is no need for an extra
check on removing the grace period after a successful upgrade.

This extra check is the reason why the automatic unlocks have recently
failed in several cases.

* refactor outgrown subscription notices

* make a test actually test the described functionality

* Apply greater pageview allowance margin only for trial upgrades

...in order to prevent cancelled or paused subscriptions from subscribing
to plans that would still leave their account locked.

* Mark the entire ChoosePlanTest module full build only

* remove account locking guide

This is irrelevant for self-hosters, and the internal knowledge base is
a better place for this document. Moved it there.

* refactor Keyword get clause

* add a pattern matching assertion in code