Adjustments to mobile dashboard layouts

It seems like our outdated pure-grid version has funny ideas about CSS loading order, and is generally confused about breakpoint handling, which caused problems on the new mobile dashboard.

So now, we hardcode a less-than-ideal breakpoint just so that it's consistent across the different grid systems. Otherwise, there can be interim states where the main content doesn't take up 100% but should, or where the navigation is placed above but still using the desktop sizing (resulting in a width that's way too small).

All in all, pure-grid should probably get replaced with some other framework we're already using 😄

Additionally, the functionality for collapsing longer content panels was rewritten to include several bugfixes, and used on the new mobile dashboard to prevent lots of scrolling before the list of updates is reached. We should try to include this improvement with the upcoming release 🍎

Screenshots