Enforce term 'topic' instead of 'tag' in project context

What does this MR do?

Related issue: #328226 (closed)

In GitLab, you can add arbitrary topics to a project in the project settings:

Project settings > General > Topics (screenshot)

In the backend, that's implemented using the acts_as_taggable_on gem. That's okay. But unfortunately, the terms tags and tag_list are used instead of topics and topic_list in some places in the backend. This is not consistent and also brings confusion with the actual tags (git tags of project repositories).

This MR fixes this confusion by replacing all occurrences of the incorrect term tag in the project context with the correct term topic. This step was discussed in #328226 (closed) and provides the basis for implementing more ideas to boost project topics.

This change has an impact on all places where the wrong term "tags" has been used for project topics so far:

Frontend: Explore projects by topic:

• Before: http://gdk.test:3000/explore/projects?tag=topic1
• After:    http://gdk.test:3000/explore/projects?topic=topic1

API: Project entity (GET):

GET http://gdk.test:3000/api/v4/projects/3
{
    "id": 3,
    "name": "Wget2",
-   "tag_list": [
+   "topic_list": [
        "topic1",
        "topic2"
    ],
    ...
}

API: Project entity (POST/PUT):

PUT http://gdk.test:3000/api/v4/projects/3
Body:
{ 
-   "tag_list": "topic1,topic2",
+   "topic_list": "topic1,topic2",
    ...
}

🛠 with ❤ at Siemens

/cc @bufferoverflow

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