R1:764e4574d11a

Reduce responsibilities of PhabricatorPasteSearchEngine->buildSearchForm()

Summary:
Ref T2625. The specialized buildSearchForm() method has significant amounts of generic form construction responsibility right now. Lift the generic stuff above the Engine level. Also:

- Rename "users" to "authors".
- Use "users", not "searchowners" (which incorrectly includes "upforgrabs").
- No need for "set_" prefixes anymore since we do GET redirects with query keys.
- Use newer style for search stuff.

Test…