Fix for tickets #2917, #2262: user URL shortening options not being applied in non...

Fix for tickets #2917, #2262: user URL shortening options not being applied in non-web channels

common_shorten_links() can only access the web session's logged-in user, so never properly took user options into effect for posting via XMPP, API, mail, etc.

Adds an optional $user parameter on common_shorten_links(), and a $user->shortenLinks() as a clearer interface for that.
Tweaked some lower-level functions so $user gets passed down -- making the $notice_id param previously there for saving URLs at notice save time generalized a little.

Note also ticket #2919: there's a lot of duplicate code calling the shortening, checking the length, and reporting near-identical error messages. These should be consolidated to aid in code and translation maintenance.