R1:098d0d93d60c

Generate expected schemata for User/People tables

Summary:
Ref T1191. Some notes here:

- Drops the old LDAP and OAuth info tables. These were migrated to the ExternalAccount table a very long time ago.
- Separates surplus/missing keys from other types of surplus/missing things. In the long run, my plan is to have only two notice levels:
- Error: something we can't fix (missing database, table, or column; overlong key).
- Warning: something we can fix (surplus anything, missing key, bad column…