File upload endpoints

[jagerman]

Three things here:

• Implementing new endpoints so submit files, which will give them a default 1h expiry time, and take a suggested filename.
• Download endpoints (not using JSON), along with filename, etc. provided in headers.
• Add recognition of "files" parameter in the post and edit message endpoints, so that it associates any listed files that don't already have an associated posts with the new (or edited) post, and updates the expiry on the files to the server default expiry time.