Credentials now have a required CredentialType, which defines inputs
(i.e., username, password) and injectors (i.e., assign the username to
SOME_ENV_VARIABLE at job runtime)
This commit only implements the model changes necessary to support the
new inputs model, and includes code for the credential serializer that
allows backwards-compatible support for /api/v1/credentials/; tasks.py
still needs to be updated to actually respect CredentialType injectors.
This change *will* break the UI for credentials (because it needs to be
updated to use the new v2 endpoint).
see: #5877
see: #5876
see: #5805
For name and description, we'll derive these from the role_field and
content type, which is much better for lots of reasons (eg changing text
the future). Also ditched the rest of the fields comming from the
standard common base model, we didn't use them and they cost several
indexes on the table.
We have some signal handlers now that perform work that do work
automatically that we want to explicitly test in our migration path, so
we have to undo some things in order to test the migration code.
