[UX] Mobile project browser: make it easier to get to the queue

This issue was spun off from https://github.com/backdrop/backdrop-issues/issues/3092

The worst thing here is too much scrolling, combined with the fact that the installation queue is at the bottom of the page.

Original suggestion from @klonos:

Consider adding a "x number of modules added to the queue" indicator that remains sticky and acts as a "jump to" thingy.