Token user override

[peterthomassen]

Functionality to have a token act in another user's name:

1. Add a read-only nullable attribute to tokens called user_override or something (default: None)
2. Rename the user attribute to owner
3. Add generated column user which is computed as COALESCE(user_override, owner)

... and adjust FKs and token management code appropriately.

The attribute can only be set with the token owner's consent and when manage_tokens == False and when it is an API token (not a login token). Allow users to see who holds an override token for their account.