Move Gitlab::Database.read_write? check to TokenAuthenticatable

Instead of checking it in the User model, move it as deep as possible
to avoid future mistakes.

app/models/concerns/token_authenticatable.rb

@@ -43,15 +43,17 @@ module TokenAuthenticatable
         write_attribute(token_field, token) if token
       end
 
+      # Returns a token, but only saves when the database is in read & write mode
       define_method("ensure_#{token_field}!") do
         send("reset_#{token_field}!") if read_attribute(token_field).blank? # rubocop:disable GitlabSecurity/PublicSend
 
         read_attribute(token_field)
       end
 
+      # Resets the token, but only saves when the database is in read & write mode
       define_method("reset_#{token_field}!") do
         write_new_token(token_field)
-        save!
+        save! if Gitlab::Database.read_write?
       end
     end
   end

app/models/user.rb

@@ -1075,8 +1075,6 @@ class User < ActiveRecord::Base
   # we do this on read since migrating all existing users is not a feasible
   # solution.
   def rss_token
-    return read_attribute(:rss_token) if Gitlab::Database.read_only?
-
     ensure_rss_token!
   end