Some hotfixes + small Markdown reformat

[ddabble]

Proposed changes

• Fixed wrong comparison of MultiLingualTextStructure
• Fixed ContentBox.get_absolute_url() not working
• Fixed unable to submit required search select widgets
  • Which are - among other places - used in the "Add New Member" form
• Added one blank line after all Markdown headers
• Updated some requirements

Areas to review closely

Check that the requirements whose major or minor version were updated, don't introduce any errors or otherwise unwanted behavior (based on the packages' release notes, this should not be the case).

Checklist

(If any of the points are not relevant, mark them as checked)

• [x] Created tests that fail without the changes, if relevant/possible
• [x] Made sure that your code conforms to the code style guides and that any common code smells have been addressed
  • (It's not intended that you read through this whole document, but that you get yourself an overview over its contents, and that you keep it in mind while taking a second look at your code before opening a pull request)
• [x] Added sufficient documentation - e.g. as docstrings or in the README, if suitable
• [x] Added your changes to the "Unreleased" section of the changelog - mainly the changes that are of particular interest to users and/or developers, if any
• [x] Added a "Deployment notes" section above, if anything out of the ordinary should be done when deploying these changes to the server

[ddabble]

Merging without approval, as the (functional) changes have been live on the website for some weeks now (through local changes 🤠), and because the Markdown changes are nice to have before any future pull requests are created.