Break out nested relationship API keys (#5887)

* Break out nested relationship API keys

This closes #5856 by restoring the existing behavior of the `muting`
and `following` keys (returning booleans rather than truthy or false).
It adds `showing_reblogs` and `muting_notifications` keys:

* `showing_reblogs` returns true if:
  1. You've requested to follow the user, with reblogs shown, or
  2. You are following the user, with reblogs shown.
* `muting_notifications` returns true if you have muted the user and
  their notifications as well.

* Rubocop fix

* Fix pulling reblog/mute status from relationships

I could swear this had passed tests before, but apparently not.
Works now.

* More test fixes

Really, you'd expect this to be more straightforward.