Improved performance of sidebar rendering. Fixed RTL sidebar opening.

packages/rocketchat-theme/assets/stylesheets/base.less

packages/rocketchat-theme/assets/stylesheets/rtl.less

packages/rocketchat-ui-master/master/main.coffee

packages/rocketchat-ui-master/master/main.html

packages/rocketchat-ui/lib/menu.coffee

packages/rocketchat-ui/views/app/burger.coffee

packages/rocketchat-ui/views/app/burger.html