Add index on labels title

What does this MR do?

Add index on labels title

Are there points in the code the reviewer needs to double check?

N/A

Why was this MR needed?

One of the queries used in Groups::MilestonesController#show (counting number of issues per label) is doing a sequential scan on labels table, so it should help improve the loading time of that page.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [] Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

N/A