Feature/pbc requests filters autocomplete fix

Story: https://www.pivotaltracker.com/story/show/62946884

This pull request does a bit more than the story says:

1. It decouples the autocomplete init code from modals_controller so that it was easier to use within tree_filter_controller
2. I've also made a change that shows the autocomplete list on focus (instead of waiting for the first keydown). This was requested while I was working on assessments hf mockups, but it seems really useful so I'm including it here as well. Let me know if I should revert this.
3. The last commit enables autocomplete in tree_filter_controller.