Remove remaining Bootstrap classes and delete CSS asset

[jgaehring]

Replace remaining content components (eg, buttons, tabs, input fields, etc) in App.vue and in other core components.

[jgaehring]

Forget where I first stumbled across this, but I REALLY like this CSS framework, as a possible replacement for the baseline layout classes provided by Bootstrap, like .container-fluid etc:

https://picocss.com/

Having a greatly simplified drop-in replacement like this could make the whole task of removing that ~150kb Bootstrap CSS file a whole lot easier, as opposed to recreating those classes myself. That might not be necessary, but in case it is.

Even with Pico.css's 10kb that's a huge drop in the net size of those vendor bundles, maybe even lower if I can drop Normalize.css as well.