Misc tidying and clean ups (#3445)

* Remove trailing whitespace in i18n mailers

* Use query methods instead of #present? on AR attributes

* Delegate Status#account_domain method

* Delegate Mention #account_username and #account_acct methods

app/models/mention.rb

@@ -17,4 +17,11 @@ class Mention < ApplicationRecord
   has_one :notification, as: :activity, dependent: :destroy
 
   validates :account, uniqueness: { scope: :status }
+
+  delegate(
+    :username,
+    :acct,
+    to: :account,
+    prefix: true
+  )
 end