Add API endpoints to list journals and characters for a given user

[IsaacSchemm]

Endpoints for getting details on an individual journal entry or individual character were added in #20. I think it would also be helpful to provide endpoints to list all journals or all characters posted by a certain user (or at least their IDs), similar to the gallery endpoint for submissions.

I have a personal server that checks my own Weasyl account and mirrors whatever I post there, and this would be a nice way for it to be able to include any journals I post without having to do any HTML scraping.