Make adding projects a standard Herald effect

Summary: Ref T7849. Lift project actions into the base class. Some day they'll be fully modular.

Test Plan:

• Wrote an "add projects" Herald rule.
• Edited Maniphest tasks.
• Got projects added.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7849

Differential Revision: https://secure.phabricator.com/D12503