Modal tags

Storing cached modals in `<script>` tags now so they're out of the DOM until referenced (prevents collisions with IDs). Goes along with DoSomething/ds-neue#161.

:warning: Changed the ID for login/register modals to use the "modal--*" convention we'd settled on earlier. May need to be updated elsewhere these same modals are referenced. /cc @mmwtsn 

Closes DoSomething/dosomething#835.