R1:105cbaaee1dd

Implement a basic version of ApplicationEditor in Paste

Summary:
Ref T9132. Ref T4768. This is a rough v0 of ApplicationEditor, which replaces the edit workflow in Paste.

This mostly looks and works like ApplicationSearch, and is heavily modeled on it.

Roughly, we define a set of editable fields and the ApplicationEditor stuff builds everything else.

This has no functional changes, except:

- I removed "Fork Paste" since I don't think it's particularly useful now that pastes are editable. We could…