Allow pastes to be edited

Summary: Permits the name and langauge of a paste to be edited. This will eventually allow the visibility policy to be edited as well.

Test Plan: Edited name/langauge of some pastes. Tried to edit a paste I didn't own, was harshly rebuffed.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1690

Differential Revision: https://secure.phabricator.com/D3376