Make toolbar's microphone button enabled whenever there are any
'audioinput' devices available and allow to add audio during
the conference even if microphone permissions were denied on startup.

Dynamically enables/disables the toolbar video button. Prior to that
commit if we would start with no video there would be no way to enable
it later on.

Enable ESLint on jitsi-meet with the same configuration and the same
goals as in lib-jitsi-meet.