[Mentions] Create a nice combobox fox the mentions

The Mentions selector should have a great UX like a typical autocomplete/combobox. @jpuri did some great work we can copy here already: https://github.com/nikgraf/belle/blob/master/src/components/ComboBox.js

Re-creating the Facebook experience is the goal I have in mind right now. Feel free to think about higher standards.

Note: We can drop the `Dropdown` component if it helps to build it.
