Restore "download repository path" feature, now with fixes and a feature flag

What does this MR do?

We reverted !26532 (merged) in !27249 (merged) because we found a regression: https://gitlab.com/gitlab-org/gitlab-workhorse/issues/218

The cause of the regression has been identified, so this MR restores the reverted code, and adds:

• A fix to use Base64::encode64 instead of Base64::urlsafe_encode64
• A feature flag, enabled by default

If there are further problems, the feature flag will allow us to disable without another revert.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content sers](https://docs.gitlab.com/ee/install/requirements.html#supported-web-browsers)

Closes #24704 (closed)

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/218