config: switch default disableRtx to false

Replace features/unsupported-browser SET_UNSUPPORTED_BROWSER with features/base/lib-jitsi-meet SET_WEBRTC_READY

The error raised by JitsiMeetJS.init() is already in the state of
features/base/lib-jitsi-meet so it's not a good design to store the same
error in the state of features/unsupported-browser.

config: Don't require an extension for firefox >=52.

Starting with firefox 52, no extension is required for screensharing.
See https://wiki.mozilla.org/Screensharing

cleanup: remove old adaptive simulcast config option

`host` contains the hostname:port portion, whereas `hostname` is just the
hostname, not including the port.

fix(filmstriponly): Set the background to transparent

Volume slider for remote participant audio elements

Recent versions of the Android plugin for Gradle started to automatically
download the SDK build tools dependency if it is not installed already.
So it is no longer necessary to have the developer of the Android app
install the SDK build tools dependency in advance.

Move miscellaneous files to resources

Re-styles range inputs for Chrome and FF.

When rendering using a for loop each child whould have a key prop.