Issue #207: add a menu resource for ThingFragment

The plan is to convert all the menu items from ThingMenuFragment.
Issue #207: make AccountAuthenticatorActivity work

Copy AccountAuthenticatorActivity and make it extends the support
library's FragmentActivity to make it work with support fragments.
Issue #207: implement open and copy url menu items
Issue #207: add back link and comment menu items
Issue #207: add user and subreddit items
Issue #207: add share menu item
Issue #207: readd add subreddit menu item
Issue #207: keep removing ThingMenuFragment refs

This makes the menus hide the account menu items properly.
Issue #207: use ThingFragment in ThingActivity

...and get rid of ThingMenuFragment and the crud needed in LinkFragment
and CommentListFragment that was needed to support it. I still left
behind some code in CommentListFragment that I will need to refer to
when adding back the new comment and save menu items.

Going to comments first is broken by this change.