Do housekeeping. Wild mix of JS and CSS imports in base.html

[4lm]

There is a wild mix of JS and CSS imports in the base.html template file.

I will have a look and improve things where I can:

• [x] vendor files are pasted manually into folders, have no programmatic way of updating (e.g., npm based update script) and therefore will most likely never be updated - this is in part my doing in the past, hunting me now ;)
• [x] remove third-party CDN imports of assets that can be self-hosted
  • [x] fix duplicate import of Bootstrap CSS
  • [x] self-host select2
  • [x] create programmatic import update for select2
  • [x] self-host "de" select2 extension
  • [x] create programmatic import and update for "de" select2 extension
  • [x] what is django_select2? Is this an artefact? Clarify and act!
  • [x] self-host eCharts
  • [x] create programmatic import and update for eCharts
  • [x] research and act on results: does eCharts has to be imported in head?
  • [x] create programmatic import and update for jQuery
  • [x] create programmatic import and update for pubSub
  • [x] create programmatic import and update for Bootstrap
  • [x] remove unused Bootstrap Multselect artefacts
  • [x] create programmatic import and update for MapLibre
  • [x] create programmatic import and update for IonRangeSlider

I will also have a look at the map.html file.

[henhuy]

Yes please! no CDN. Cleaning is very welcome!