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.

spec/controllers/api/v1/accounts/relationships_controller_spec.rb

@@ -32,7 +32,7 @@ describe Api::V1::Accounts::RelationshipsController do
         json = body_as_json
 
         expect(json).to be_a Enumerable
-        expect(json.first[:following]).to be_truthy
+        expect(json.first[:following]).to be true
         expect(json.first[:followed_by]).to be false
       end
     end
@@ -51,7 +51,8 @@ describe Api::V1::Accounts::RelationshipsController do
 
         expect(json).to be_a Enumerable
         expect(json.first[:id]).to eq simon.id.to_s
-        expect(json.first[:following]).to be_truthy
+        expect(json.first[:following]).to be true
+        expect(json.first[:showing_reblogs]).to be true
         expect(json.first[:followed_by]).to be false
         expect(json.first[:muting]).to be false
         expect(json.first[:requested]).to be false
@@ -59,6 +60,7 @@ describe Api::V1::Accounts::RelationshipsController do
 
         expect(json.second[:id]).to eq lewis.id.to_s
         expect(json.second[:following]).to be false
+        expect(json.second[:showing_reblogs]).to be false
         expect(json.second[:followed_by]).to be true
         expect(json.second[:muting]).to be false
         expect(json.second[:requested]).to be false