Create interface and permissions for managing scheduled topics
Scheduled topics are working, but currently have to be created "manually" through python or the database. There should be an interface for configuring them through the site, both creating new ones and editing existing ones. This should also require a specific permission, probably granted manually.
Important note: both the title and markdown for scheduled topics are treated as Jinja templates. It uses Jinja's sandboxed environment, but may still not be completely safe. It might be best to implement a full whitelist-based approach for this, so that only specifically approved attributes/methods can be used on the variables. Probably needs some more research and should be treated carefully before giving more people access to scheduled posts.