interface methods: create, consume, list and revoke.
SQLite implementation and some light testing.
Related changes:
* have authfallback.Create return the user id
At some point we will need to not assume that authfallback is our users
table but that will not become relevant before we start adding
moderation roles.
* Update package documentation of admindb and admindb/sqlite
* remove leftover generated.db
now using the roomdb file created by TestSimple
Review comments by @cblgh
* better documentation of hashed token storage
* space between %d and `bytes`
* make interface assertion comments less scary
Henry