Implement gitlab:db:deduplicate_tags rake task

What does this MR do and why?

This MR formalizes the $3700665 script into a rake task that users can run if they suspect there are duplicate tags in their database.

Due to OS upgrades bringing differences in the underlying index collation provided by libc, it can happen that duplicate tags are allowed into the tags table. There is a snippet that has been used by users to fix these problems in an ad-hoc fashion. With this new rake task, we can be sure that the logic being executed matches the current version of the database as it evolves, and that it is tested.

The runner-side documentation is being updated in gitlab-runner!5356 (merged).

References

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• Create rake task to deduplicate runner tags (#518698 - closed)
• $3700665
• Update documentation to point to Rake task to d... (gitlab-runner!5356 - merged)

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