Remove the concept of sources

[linknum23]

After creating a working example of #262 we started thinking of reworking the source/stream concepts in the system to handle background streams. Below is an "off the cuff" TODO list.

• [ ] remove all sources
• [x] #353
• [x] #332
• [ ] replace streams with the concept of an active stream, will this be the place where info moves?
• [x] split the stream.connect/disconnect concept into activate/deactivate, connect/disconnect where connect connects a loopback device or RCA device to one of the 4 sources and activate starts a stream or background stream
• [ ] replace source_id field in groups and zones with stream_id?
• [x] #330
• [ ] add some dynamic event handling that would happen (in example) when a user starts playing to 'Office' Spotify device

On the UI side we will need to start looking into:

• [ ] #211
• [ ] Active stream management cmds={restart, activate, deactivate} streams

[rickballard]

This is a very good idea

[linknum23]

Initial work on this is being done in https://github.com/micro-nova/AmpliPi/tree/virt-streams which allows streams to be run in the background.