New log to join meetings from mobile devices

Now there's no specific page to join from a mobile. Once /join is called,
if the user is in a mobile device (detected using the user's "user-agent")
then a page will be rendered and the user will be automatically redirected
to the conference using the mobile client. This page has also more information
just in case the user is not properly redirected (if the mobile client is
not installed, for example).

Also, there's no more login via QR Code. It wasn't a good solution and was not
being used, so it was simply removed.
Merge branch 'master' into branch-v1.4.0-mobile

Conflicts:
	Gemfile.lock
Add tests for the latest changes in the mobile join
Update gem browser to 0.5.0

refs #1247
Small change in a variable name in RoomsController#join_mobile

refs #1247
Merge remote-tracking branch 'origin/branch-v1.4.0-mobile'