Optimise upload path calls

What does this MR do?

String#underscore isn't particularly slow, but it's possible for us to call it many times in a users autocomplete request, with mostly-static values ('User', 'Group', etc.). We can memoise this and save a surprising amount of time (around 10% of the total request time in some cases).

See https://gitlab.com/gitlab-org/gitlab-ce/issues/60903#note_168567352.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.