R1:33ec855449dd

Modularize application transactions in Paste, mostly

Summary:
Ref T9789. `Transaction` and `Editor` classes are the last major pieces of infrastructure that haven't been fully modularized.

Some of the specific issues are:

- `Editor` classes rely on a bunch of `instanceof` stuff in the base class to pick up transaction types like "subscribe", "projects", etc. Instead, applications should be adding these, and third-party applications should be able to add them.
- Code is spread across `Transaction`…