Dropdowns Roadmap: List features

In order to choose an external lib to use we need to list all the features we need a dropdown to have.

Have in mind that we'll probably end up with 2 dropdown solutions:

1. A basic one like bootstrap with almost no javascript
2. A more advanced one with search, ajax calls, etc

This issue is to list the more advanced features we need.

Main issue is: https://gitlab.com/gitlab-org/gitlab-ce/issues/39971