Currently, Maniphest supports search by project (including not/all/any) and Diffusion supports this in a limited way.
All applications with objects which implement `PhabricatorProjectInterface` should support reasonable search-by-project capabilities. This should be implemented in a general way. Rough approach here is:
- Move Maniphest to modern edges (T5245).
- Modernize typeaheads (T4420).
- Parameterize typeaheads (T4100) and use that to reduce the UI weight of project filters (by combining fields using parameters).
- Generalize search in Maniphest.
- Port and generalize search in Diffusion.
- Finally, support general search for all `PhabricatorProjectInterface` objects.