Add 'Explore topics' page

What does this MR do?

In GitLab, you can add arbitrary topics to a project (in the project settings). But as far as I know, these topics are not yet so well known. So let's give a boost to the use of project topics!

This MR makes it possible to explore topics using the new Explore topics page in the Projects area (http://gdk.test:3000/explore/projects/topics).

• The topics are sorted by popularity (number of projects with this topic). For database performance reasons no other sorting is available. This would also not be a common use case.
• The topics can be searched by name and are then sorted by similarity (and by popularity).
• By clicking on the desired topic, you will get to the Explore projects page containing the corresponding projects of the topic. Once !71830 (merged) is merged, the link should redirect to the details page of the corresponding topic instead.

🛠 with ❤ at Siemens

/cc @bufferoverflow

Screenshots

Outdated videos

1. version:

Project_topics

2. version:

Project_topics_new

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team