Proxy views: use safe join
Add send_file_partial implementation, copied from the internet

http://blog.asgaard.co.uk/2012/08/03/http-206-partial-content-for-flask-python
send_file_partial: changes from the blog post comments

Off-by-one change plus path checks
send_file_partial: add mimetype param

Also make the HTTP status code a constant
Proxy views: use send_file_partial for MP3s