Preliminary mobile styling

[ysulyma]

Improves the basic UI on phones.

numberscope.mobile.mov

• installed @phosphor-icons/vue to make the icons
• fixed the isMobile() function (the --ns-breakpoint-tablet CSS variable was not available due to incorrect SCSS interpolation)
• made the sequence/visualizer tabs pop open from the bottom

Minor changes

• changed visualiserTab to visualizerTab for consistency
• spec-name should be a class instead of an ID since it appears twice (desktop and mobile)
• removed unnecessary #navbar-main div

Not included

I have not:

• done any work on the gallery, sequence chooser, etc. (anything not shown in the above screengrab)
• tested on an actual phone, only in the browser's mobile emulation mode