Paste - add transactions

Summary: Ref T3650. This adds a create transaction, transactions for metadata (title, langauge, view policy), and comments. Editor is used on all create /edit paths.

Test Plan: made some pastes via web and email - yay. edited pastes - yay. verified txns showed up on pastes and in feed correctly.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3516, T3650

Differential Revision: https://secure.phabricator.com/D6645