This adds the backend for scheduled topics, which can be set up to post
at a certain time and then (optionally) repeat on a schedule.
Currently, these topics must be text topics, and can have their title,
markdown, and tags set up. They can be configured to be posted by a
particular user, but if no user is chosen they will be posted by a
(newly added) generic user named "Tildes" that is intended to be used
for "owning" automatic actions like this.