[#2349] Made :skip_plug/2 prevent plug from being executed even if explicitly called. Refactoring. Tests.

This reverts merge request !1877

The two changesets had the same purpose, yet some changes were updated
in one, but not the other (`uri`, for example).

Also makes `Transmogrifier.upgrade_user_from_ap_id` be called from
`ActivityPub.make_user_from_ap_id` only when the user is actually
not AP enabled yet.

I did not bother rewriting tests that used `User.insert_or_update`
to use the changeset instead because they seemed to just test the implementation,
rather than behavior.

Unlike concatenating strings, this makes sure everything is escaped.
Tests had to be changed because Phoenix.HTML runs attributes through
Enum.sort before generation for whatever reason.

Apply suggestion to test/web/mastodon_api/controllers/account_controller/update_credentials_test.exs

Enforcement of OAuth scopes check for authenticated API endpoints, :skip_plug plug to mark a plug explicitly skipped (disabled).

[#2323] Fixed a typo causing /accounts/relationships to render default relationships. Improved the tests.

Ability to control the output of account/pleroma/relationship in statuses in order to improve the rendering performance.
See `[:extensions, output_relationships_in_statuses_by_default]` setting and `with_relationships` param.

FollowingRelationship storage & performance optimizations (state turned `ecto_enum`-driven integer, reorganized indices etc.).