Refactor user groups into organizations

[henhuy]

Summary of the discussion

• This PR refactors user "groups" into "organizations". Renaming of models is done via migrations and presences in code are adapted as well.
• New fields "acronym", "image" (per link) and "homepage" (link) are added to organization model.
• HTMX flow on organization views and templates is streamlined

Type of change (CHANGELOG.md)

Changes

• Refactor user groups into organizations;
  add fields to the organization model;
  refactor HTMX for organization management pages

Workflow checklist

Automation

Closes #2262

PR-Assignee

• 🐙 Follow the workflow in
  CONTRIBUTING.md
• 📝 Update the
  CHANGELOG.md
• 📙 Update the documentation on
  mkdocs

Reviewer

• 🐙 Follow the
  Reviewer Guidelines
• 🐙 Provided feedback and show sufficient appreciation for the work done