Add file upload functionality

Adds an :allow_uploads wiki option, an --allow-uploads flag to
bin/gollum, an "Upload" button with a file upload dialog, and a
handler to commit uploaded files into the repository.

:allow_uploads defaults to false, to prevent unauthenticated users
from uploading arbitrary files into the repository (albeit only in
the uploads directory).

This code is based on the patch from @l3iggs at
https://github.com/gollum/gollum/issues/694, but the handling on the
backend is completely rewritten to use the Committer infrastructure.