The Client object on its own doesn't fully express everything about a
single client, and so when loading clients from a static configuration
it's not enough to just (de)serialize clients.
To that end, LoadableClient contains the full representation of a client
and associated entities.
Replaced by ClientRepoFromClients, which makes more sense IMO. Also, it
was doing the wrong thing: it was ignoring the client_id and client_secret
passed into it as far as I can tell.
The DB implementation expects secrets to be base64 encoded blobs.
Because of this a bunch of tests broke moving to sqlite.
A lot of this commit is fixing those tests.
