Left-menu mobile styles as a breakpoint

Instead of relying on the isMobile property from the state driven server-side from the user-agent, we just decide this based on the size of the viewport (breakpoint).

This helps with people who use Gitter at a smaller size on desktop. The left-menu will just be out of the way when someone page-loads at a small viewport size (especially true for NLI).

• Mobile: @media (max-width: @left-menu-mobile-breakpoint) {
• Not mobile: @media not all and (max-width: @left-menu-mobile-breakpoint) {
• Also added checkMobileBreakpoint() -> public/js/vue/left-menu/mixins/check-mobile-breakpoint.js for use in the JavaScript

Part of https://gitlab.com/gitlab-org/gitter/webapp/issues/2437

Signed in

.	Above 900px	Below 900px	Below 900px expanded
After:
Before:			Pinned:
.	.	.	Unpinned:

Not logged in(NLI)

.	Above 900px	Below 900px	Below 900px expanded
After
Before			N/A, always expanded

Testing strategy

• Above 900px
  • Signed in
  • NLI
  • Pinned
  • unpinned
• Below 900px
  • Signed in
  • NLI