Closes #42

Rewrote handling of attachments:

- New file types are supported: ZIP archives, JPEG and PNG.

- Types are automatically detected using python-magic, known extensions
  are preserved. Types are stored in the database.

- Attachment size limit (which BTW did not work) made configurable.

- UTF-8 detection rewritten to be scalable.

- Links to download attachments now contain cryptograpically signed
  post ID, user ID, and recommended filename (for student's submissions,
  it's based on the student's name).

Heavily inspired by a similar hack in OSMO.

Closes #45, this time hopefully for real.

Otherwise, big sets of cookies sometimes do not fit and the server
fails with error 500.

(caused by recent refactoring)

Closes #43.

Closes #37.

Closes #50.