Add notes to blocks (#193)

In this commit, we also push note creation into the AccountInteractions
concern, where we can:

1. create and persist model objects inside transactions without having
   to include job queuing inside the transaction
2. commonize the don't-add-a-blank-note check

Finally, this commit resolves a invocation-on-nil in
REST::RelationshipSerializer.

app/controllers/api/v1/accounts_controller.rb

@@ -21,7 +21,7 @@ class Api::V1::AccountsController < Api::BaseController
   end
 
   def block
-    BlockService.new.call(current_user.account, @account)
+    BlockService.new.call(current_user.account, @account, note: params[:note])
     render json: @account, serializer: REST::RelationshipSerializer, relationships: relationships
   end