Make _BACKGROUND_BLUR temporary flag permanent and bring it to stable branch

I have been working in making jitsi-meet working at acceptable speed with a Raspberry Pi 3, something that I successfully accomplished by:

1. Implementing ffmpeg with h264 hardware acceleration enabled for the Pi.
2. In Chromium web browser, go to chrome://flags and enable #enable-webrtc-h264-with-openh264-ffmpeg
3. Configure jitsi-meet webcam capture restrictions to 480p

With these steps, jitsi-meet can work quite well in a Raspberry Pi 3 if _BACKGROUND_BLUR is set to off. It looks like the blur process takes a significant performance hit in a limited device. Hence, I consider it would be important to keep this option permanent or make off the default setting.

[virtuacoplenny]

I ran it by Emil and others and the sentiment is to keep the flag. I might have to change the name though, maybe rename it to DISABLE_BACKGROUND_BLUR but at least remove the underscore prefix.

This was good timing because I already made a PR to clean up this flag.

[cmilanf]

I don’t recall the Chromium version I used, but it was definitely the one in Raspbian Stretch. You have several versions to select in the same distro. If latest one, 68, does not have the flag, maybe you can use 57.