Fix vsa label dropdown limit

What does this MR do?

Fixes #212895 (comment 314203285)

UI

• Consolidates the label dropdowns for VSA
• Adds the search box to both dropdowns
• Fetches the first 20 group labels (no ancestor / descendent or project labels)
• Refetches data on search

Data

• Handles fetching new labels when searching
• Removes all vuex store related code for group labels, since they are no longer stored in state to be shared
• Replaces the /api/:version/groups/:namespace_path/labels with /groups/:namespace_path/-/labels.json to allow use of the search parameter

Screenshots

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