Add JS to show/hide 'Selected' message on add-to-set modal

This is modified from the original version in the UI repo. Here we just
add/remove a selected class on the <li> for each set. The
.set-is-selected class is made visible/hidden based on this.
Wire up edit-post js from UI

Wiring up js to replace title/content with input and textarea fields.
This is broken.. but we can fix it when we start dealing with real
content.