Split language detectors to be web/native dependent. Take care of strings that contain html.

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.

Until we make a decision on access modifier hints and adopt a respective
coding style, consistency is king.

The desired behavior of the button 'Start a conference' / 'Join the
conversation' is to launch the mobile app if installed; otherwise, do
nothing i.e. continue to display UnsupportedMobileBrowser.

Anyway, we may change our minds about allowing the user to continue in a
supported mobile browser so preserve the source code that enables that
but give it more appropriate naming.

React Native provides a Platform abstraction which React does not
provide.