R1:92ff9c092b25

Move "Macros" to a first-class application

Summary:
This is mostly to unblock D3547.

- Move "Macros" to a first-class application called "Macros".
- After D3547, this application will also house "Memes" (macros with text on them).
- This will also make them easier to find; the top navigational query I field is "where are image macros?" nowadays, since it's not intuitive they're part of files.
- This makes some of the UI mobile-aware but doesn't set the `device` flag yet, since there are still…