Page for admins to edit user mappings

[jmgasper]

We need an admin-only page that allows users to edit user mappings. This will make it easier to debug and fix issues if a git link doesn't work for some reason. We can add this new page in the left menu, for admins only, called "User Mappings"

This page should allow ADMINS ONLY to:

1. View all mappings of a Topcoder member name to their Gitlab and Github user names and user IDs
2. Search / filter for a specific Topcoder member name or Gitlab / Github username
3. Add a new mapping for a Topcoder member name to a Gitlab and/or Github username (either both or one or the other)
4. Delete a mapping for a Topcoder member name
5. Update a mapping