[FastAPI] PackageBase actions

The actions panel on the right-hand site of /pkgbase/{name} (or /packages/{name}) needs to be fully functional.

With kevr's pu_packages_actions branch, list-wise forms of these actions already exist (for one or more Packages). Let's derive from those, or decouple the single logic into functions we can re-use for the actions panel.

NOTE: Most of this issue has already been addressed in merged commits. There is one outstanding action, which is not directly tied to pkgbase actions: pkgbase merge, which an MR is currently up for.