Project Topics: let's give a boost

In issue #328226 (closed), the project topics were detached from the tags. After this preliminary work, it's finally time to give project topics a boost!

So I will implement the following new features as Community contribution via separate MRs:

1. Show topics in project lists: !65335 (merged) & !67825 (merged)
2. Add avatar and description to project model: !70386 (merged)
3. Add topics management (admin area): !71033 (merged) - #345645 (closed)
4. Add Show topic page: !71830 (merged)
5. Add Explore topics page: !62443 (merged)
6. Make topics selection in project settings more user-friendly (#341896 (closed)): !72299 (merged)
7. Add Topics API: !73969 (merged)

This issue will be used to coordinate this work.

🛠 with ❤ at Siemens

/cc @bufferoverflow