R1:d7693a93b3c4

Provide "Change Projects" and "Change Subscribers" (instead of "Add ...") in comment actions

Summary:
Ref T9908. Fixes T6205.

This is largely some refactoring to improve the code. The new structure is:

- Each EditField has zero or one "submit" (normal edit form) controls.
- Each EditField has zero or one "comment" (stacked actions) controls.
- If we want more than one in the future, we'd just add two fields.
- Each EditField can have multiple EditTypes which provide Conduit transactions.