Make get_mod_mail usable by Subreddits

Also renamed moderator in API_PATHS to mod_mail to make
it more clear what it represents.
Fix bug in mod-only meths with subreddit default

In @restrict_access the first element in args was used to
determine the subreddit for Reddit methods. However, this
would not work when  the subreddit was not given as an
argument and the default used instead, in this case args
would be empty.