RFC-0040 License Package Sources: implement warnings and AUR page overview (step 1: grace period)

In line with RFC-0040 https://rfc.archlinux.page/0040-license-package-sources/ we should implement appropriate first steps to ensure we minimize pain and friction for future interaction between AUR packages promoted to the official repositories.

As first step, this ticket proposes to:

  • Issue a warning on every git push if the repository is lacking the required license declarations
    • include a link and advice how to collect the required permissions and how to setup the correct files
    • create an easy facility to potentially reject future pushes if such a license is missing
  • Display a property on the AUR web page indicating if this packaging repository has an appropriate license
    • display some green status if its in line with RFC-0040
    • display a visual warning if it isn't
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