R1:bf17b12daf43

Standardize SSH key storage

Summary:
Ref T5833. This fixes a few weird things with this table:

- A bunch of columns were nullable for no reason.
- We stored an MD5 hash of the key (unusual) but never used it and callers were responsible for manually populating it.
- We didn't perform known-key-text lookups by using an index.

Test Plan:
- Ran migrations.
- Faked duplicate keys, saw them clean up correctly.
- Added new keys.
- Generated new keys.
- Used `bin/auth-ssh` and `bin/auth-ssh…