This change introduces client API tokens which are API tokens that belong

specifically to a given client. Clients (e.g. RBTools) will use these tokens

to authenticate to Review Board. These tokens are important for an upcoming

change that allows users to authenticate clients to Review Board via a

web-based login flow.
During the web-based login flow, client API tokens are automatically created

if they do not exist or if previous client tokens are expired. Each client

will have their own client tokens, and the tokens will be marked with a note

that says the token was automatically created for the given client.

Ran unit tests.
Tested creating a normal token and saw the right email being sent.
Tested creating a client token and saw the right email being sent.
Manually tested with the upcoming client web-based login flow change.