Give AphrontTagView a getViewer(), deprecate getUser()

Summary:
Two minor changes here:

• Replace get/setUser() with get/setViewer() for consistency with everything else.
• getViewer() now throws if no viewer is set. We had a lot of code that either "should" check this but didn't, or did check it in an identical way, duplicating work. In contrast, very little code checks for a viewer but works if one is not present.

Test Plan:

• Grepped for ->user.
• Attempted to fix all callsites inside *View classes.
• Browsed around a bunch of applications, particularly Calendar, Differential and Diffusion, which seemed most heavily affected.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15412