Add responsiveness for mobile devices

I love Rainloop for being a modern and great app, I've often felt it lacked a mobile view. To that end I've created a new Responsive.less to apply styles to make Rainloop pleasant to use on very narrow screens such as mobile phones.

I've deliberately kept the styles self-contained, and wrapped in a html.ssm-state-mobile selector, which avoids any potential for unexpected problems on larger views. It also makes use of this a little more optional