Adds configuration option web/REPORT_ERRORS_TO.

* Message is shown on every 500 page.
Added support for MAX_CONTENT_LENGTH option.

* This is known not to work correctly with nginx. However, this commit
  does everything that is possible on the application's side.
Merge branch 'master' into v0.2dev

Conflicts:
	galah/base/config.py
	galah/web/views/_upload_submission.py
	galah/web/views/_view_assignment.py