Projects rework to automatic "Related resources"

It is proven by now that people don’t do the manual picking and connecting of projects. Nextcloud should be intelligent enough to show related folders, Deck cards, events, conversations, etc. automatically.

The idea is to show folders, Deck cards, calendar events, conversations, etc. which are shared with the same group or circle. This should give useful links to other resources in 90% of the cases without manual interaction.

Proposal

• The manual "Projects" entry is expanded into a "Related resources" (wording not set, but nicely alliterated) section below the share list.
  • Alternatively, and if we actually introduce a "Details" tab at some point, we could also think about moving this "Related resources" section over there. This might be needed cause the sharing list can grow quite long.
• There is an explanatory subline "Anything shared with the same group of people will show up here."
• Below is a list of the related resources. They have sublines with relevant info like last message for Talk conversation, column and board for Deck cards, or dates for Calendar events.
• If any of the entries is clicked:
  • First step: Just like for Dashboard, you get redirected to that relevant item, opening it in a new tab.
  • Ideal future (also just like ideally for Dashboard): A simple modal opens with further detail on that element only, making for much smoother UX than a redirect and full loading of an entire page.

Questions / beyond minimum viable product

• How would it show up in other apps:
  • In Talk it would have to show up in the "Details" tab
  • In Calendar in either "Details" or a new tab
• Can people dismiss entries which are not fitting? (Probably not in the first version)
• How can someone see all the resources of a "Project", if that even exists?
  • Initially, any of the sidebar entries will do, as all of them should show the same.
  • Additionally, we could show the entries in Contacts/Circles in the relevant section. (Not really possible for groups)

cc @nimisha-vijay @AndyScherzinger @skjnldsv as discussed. :)

No files, tests, or entry points are named. Start by mapping the existing Projects and share-list implementations across the server and client apps, then clarify the MVP boundaries and validate the related-resource behavior with maintainers; done means an agreed scope and implementation plan.