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 Plan:
Searched for stuff?

{F44342}

Reviewers: btrahan, blc

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6061