Improve repository tree 1,000 item limit

Zendesk: https://gitlab.zendesk.com/agent/tickets/89540

Follow up to: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15333

We implemented a 1,000 item limit in the repository tree viewer recently to address performance issues for large repositories. Now that we have taken care of the immediate issue let's circle back and make sure we have the best long term solution.

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15333 we basically asserted that the 'Find file' or 't' shortcut were enough for users to browse to files if they weren't displayed within the 1,000 file limit. However, some users have it in their workflow to use the browser's Ctrl-F search to find files more quickly.

Potential solutions

• Frontend implements some type of asynchronous or autoload on scroll.
• Make the 1,000 file limit configurable
• Other?