Play music from a USB stick plugged into the speaker

[JRpersonal]

Play music files from a USB stick plugged straight into the speaker, as an extra source next to Bluetooth and AUX, without a media server. The speaker's Linux already reads a stick (STR's install uses it), and STR's stream proxy on the speaker could serve the files to the player. Open points: which models have a usable USB port, file formats, and browsing the stick from the app and the phone remote. From #385.

Vote with the upvote arrow if you want this; votes help me decide what comes next. Full backlog: #390