Add support for rendering section dividers in tables; use section dividers for changes on tasks

Summary: Depends on D20883. Ref T13440. In most cases, all changes belong to the same repository, which makes the "Repository" column redundant and visually noisy. Show repository information in a section header.

Test Plan: {F6989932}

Maniphest Tasks: T13440

Differential Revision: https://secure.phabricator.com/D20884