R1:6fac9044632b

Modularize Dashboard transactions

Summary:
Depends on D20399. Ref T13272. I'm moving toward fixing all the "moving panels around on Dashboards breaks the entire world" problems.

On the way there, modularize Dashboard transactions.

Test Plan:
- Created a new Dashboard.
- Edited all fiedls of a dashboard.
- Archived/restored a dashboard.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20402