0.8.0 Release Candidate 4

Changes:

• Handle missing cache files rather than waiting indefinitely
• Fix the /events/<id>/thumbnail.jpg endpoint when event in progress
• Add a /events/<id>/snapshot.jpg endpoint that works during and after events
• Rate limit event update notifications published to frigate/events to once per 5s
• Beautiful new docs (thanks @paularmstrong)
• If the detection process stops, exit the entire container rather than try and restart within the container

Images:

• x86: blakeblackshear/frigate:0.8.0-rc4-amd64
• x86 w/ nvidia (for ffmpeg only): blakeblackshear/frigate:0.8.0-rc4-amd64nvidia
• Raspberry Pi 3/4 (32bit): blakeblackshear/frigate:0.8.0-rc4-armv7
• Raspberry Pi 4 (64bit): blakeblackshear/frigate:0.8.0-rc4-aarch64