implement token auth models

[willkg]

We want to have an API for the IRC bot as well as for users to do their own thing. This requires two types of tokens:

1. site-wide tokens that can post for everyone
2. user-centric tokens that can only do things on behalf of the token-owning user

The current standup has one single site-wide token, but I think it's prudent for us to allow for a bunch of them and have them tied to a specific use (irc bot, slack bot, etc).

The current standup does not have user-centric tokens. I think that's a mistake. I think it'd be a lot easier if we could write our own standup clients and for that we need user-centric tokens.

This issue covers implementing token authentication models. I think we want to store user tokens in the StandupUser model. We want to store site tokens in their own model which hasn't been implemented, yet.

[willkg]

I implemented a SystemToken in PR #192. That covers item 1.

I'm pushing item 2 off to a separate issue because we don't need it for the MVP and it involves some additional things like permissions or something like that.

[willkg]

Issue #199 covers user-centric tokens. Closing this one out.