More detailed licensing information

[jorsn]

It would be helpful to include more detailed licensing information for all apps: E.g., AGPL-3.0-only or AGPL-3.0-or-later is a difference (I used spdx license codes, here).

See NixOS/nixpkgs@c317dce for an example where this creates problems. In NixOS, the purpose is to package the apps so that Nextcloud can be declaratively configured and automatically deployed with particular apps.