Initial DSC & Django update

[tymees]

This PR lays the foundations to built V4 upon.

Note: this PR targets the major/4 branch for now. This is a fork of develop made at the same time as the source branch, to have a clear diff. New changes in develop will need to be merged into there as well.

It's a rather large PR; I've tried to make proper commits for individual changes, to help reviewing. I've also added some comments over adding them here, such that the comment is located near the relevant code. Also, please ignore the changes in the docs folder. Those are caused by my bulk-refactors.

Highlights:

• Update to Django 4.2
  • I think I got this all sorted, but there might still be some problems lurking in the later form pages
• Updated to DSC 3
  • New app-specific base template (and moved all pages to it)
  • New form-specific base template (and moved the first form + WMO forms to it)
  • New home-page design. (Needs sign-off from desiree)
  • Initial automatic CSS migration pass (renaming classes to their new equivalents mostly)
  • Ported form-JS utils to new form (non-table-based) layout

Still todo: Well, a lot. A non-exhaustive list:

• Layout polishing pass
• All other form pages need to be converted to the new form-base-template
  • In addition, some pages might need to be split up to be less intimidating
• All other pages need to be (re-)reviewed.
• A proper stepper implementation
• New overview pages using UU-List. (Ideally we'd cut down on the amount of those pages in favour of filters)
• Everything else in https://github.com/DH-IT-Portal-Development/ethics/milestone/3
• And probably stuff I'm forgetting

[tymees]

Thanks for the review!

I've added a commit that refactors all uses of uil_filters.

There are probably more of these 'well you could've done that in bulk' problems still living in other places; I've only done those I was aware of, and those I found during my work on the pages I gave individual attention.

Normally we go with the 'one approval is enough' strategy, but I really want @miggol's review before merging, as it's quite an impactfull PR.