R1:aaab1011e5a4

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…