can_permanently_delete field in Post and TopicViewDetails serializers
cannot use Guardian's can_permanently_delete beause their use is
different. The field from the serializers is used to show the button
and the button is shown even if the post cannot be removed forever
because not enough time has passed since it was first deleted. The
guardian method is used by the controller to check that the post can
really be deleted.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
If a group you're a member of is invited to a PM, you can no longer remove yourself from it. This means you won't be able to remove the message from your inbox, and even if you archive it, it'll come back once someone replies.
