Upgrade to Bootstrap 5

[oliverroick]

Implements #45

• Updates form CSS classes, so we're using more Bootstrap CSS classes, specifically for error messaging. This allows us to remove some of the custom classes.
• Update typography: Use h2 elements instead of custom label heading

[yuvipanda]

This is great! We should hold off until jupyterhub 5 is rolled out to the veda hubs, as we're currently still on jupyterhub 4.x

[GeorgianaElena]

Thank you @oliverroick ✨ ! Do you want to fix the conflicts in this PR so we can merge and try this out in a JupyterHub 5 setup?

[oliverroick]

@GeorgianaElena I fixed the failing tests. This should be good to go now.

[yuvipanda]

Unfortunately it looks like Bootstrap 5 will now automatically switch to 'dark mode' if your system is using dark mode, and our selection CSS looks pretty bad on it:

[yuvipanda]

I fixed some of them but not all. I'll try get back to this tomorrow.

[yuvipanda]

The titles of the dropdown items are still the wrong color, and I'm not sure what css selector those are.

[yuvipanda]

Alright, all fixed!