move ponder writes into editors; better search indexing

Summary:

• Moved the remaining ->save() calls into editors
• for the PonderQuestion, separate attachRelated (needed for indexing and viewing) from attachVotes (needed for viewing but not indexing)
• Update the indexer to include comment and answer content in the search index.

Test Plan:
Stuff works as before. Also: add a comment or answer with some
easily-identifiable text and search for it; observe that
it gets indexed appropriately.

Reviewers: epriestley, nh, vrana

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1869

Differential Revision: https://secure.phabricator.com/D3654