Add OAuth endpoint / token support for API

[AlexVasiluta]

Currently, in order to access the Kilonova API, you need to use the session id token from a user. In order to facilitate future partnerships, we need to add an OAuth endpoint and support for apps / permissions so others can do actions such as sending submissions or viewing problems on the user's behalf. Since this is a big change, we will probably need to split off the new API into a v2 one.

We should also limit less used endpoints and prepare them for the v2 API.