1841 Add tooltip to default assignee selection

[galethil]

Checklist

• [x] I have followed (at least) the PR section of the contributing guide.
• [x] I fixed all necessary PR warnings
• [x] The commit history is clean
• [ ] The E2E tests are passing
• [x] If possible, the issue has been divided into more subtasks
• [x] I did a self review before requesting a review from another team member

Description

How to test

1. Open any project
2. Open subproject create dialog
3. Check if tooltip next to "default assignee" select is working properly
4. ...

Closes #1841