Allow admins to merge topics [frontend]

What does this MR do and why?

This MR is the second step of #366884 (closed):

• Implement the frontend (button and modal view on Admin Area > Topics page) to allow admins to merge topics

The backend was already implemented in a previous MR and the Topics API will be extended in a follow-up MR (see implementation plan).

Note: We already had some discussions about the UI in the issue: #366884 (comment 1019453813)

🛠 with ❤ at Siemens

/cc @bufferoverflow

Screen recordings

Merge_Topics

How to set up and validate locally

1. Navigate to Admin Area > Topics: http://localhost:3000/admin/topics
2. Add two topics (topic1, topic2)
3. Assign some projects to topic topic2 (general project settings)
4. Merge topic topic2 into topic topic1
5. Verify that topic topic2 is removed and its projects are now assigned to topic topic1

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.