Add "mark committed" action to the web UI

Summary: When a revision is accepted, allow users to manually mark it committed if there's a daemon/tracking problem. This shouldn't happen in most installs but we have less-robust support for Mercurial and some installs may not be fully configured.

Test Plan: Marked a revision committed.

Reviewers: btrahan, Makinde

Reviewed By: Makinde

CC: aran, epriestley

Maniphest Tasks: T948

Differential Revision: https://secure.phabricator.com/D1808