Improve Plinth Apps and Configuration pages for mobile devices

Created by: JosephKiranBabu

Browser: Firefox for Mobile OS: LineageOS (Android)

Plinth on mobile looks awkward with the App/Configuration listing and the description section appearing in the same vertical column. Also, on clicking one of the apps or configuration items, the page doesn't scroll to the bottom and show the corresponding entry's description.

I suggest we have a hamburger menu on the left side like on the mobile website of readthedocs.io. Suggestions for alternatives are welcome!

This feature will be significant soon, as we will launch Plinth web interface in a web view in the FreedomBox Android app for users with admin privileges. https://github.com/freedombox/android-app/issues/7