Resolve "Protected branches drop down does not show all users"

This MR refactors the implementation introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/581

Issues like #919 makes a refactor needed since the dropdowns were becoming too complex to maintain.

- Now the related dropdowns are based directly from `ProtectedBranchAccessDropdown` and its internal state is handled with JS only unlike its previous implementation that was a mix of JS and HTML inputs.
- Now selected users are appended making them always visible.

Closes #919

See merge request !687