Admin interface: keyword list improvements
  - pagination
  - display the user name, who created a keyword term
dev/reset: add db connection hint, formatting
doc: add section on PDFs, pdf.js and upgrading it

also move Frontend.md to Developer-Guide.md
Admin interface: remove drag and drop interaction from meta terms related to a meta key
Admin interface:
- ability to set special sets
- display a proper notice, when special set has not been updated
Add media set improvements related to ownership transfers and deletion