Make notification sound configurable

[pylorak]

Rainloop currently offers to play a notification sound on new message arrivals, but the sound is fixed and built-in to Rainloop. I would like to be able to customize this sound. Ideally Rainloop would allow selecting between built-in and custom sounds, and if a custom sound is selected, the user should be able to upload his own sound file.

[iscottz]

Find /rainloop/v/1.13.0/static/sounds/new-mail.mp3. Upload a different sound with the same name and replace the original with the new one.