Add mutually exclusive key-value labels

What does this MR do?

Add mutually exclusive labels

If a label title contains :: then everything until last :: (greedy matching) is considered as a label 'key' and labels with the same key are mutually exclusive:

• if multiple labels with the same key are added, only last selected is added, also any existing labels with the same key are removed
• when removing an exclusive key, and multiple exclusive keys are assigned (from before we introduced exclusive keys), all other exclusive keys are kept

Frontend TODO:

• Replace https://docs-link with an actual link to the Documentation.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9175

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer