Reportback modal

* Opens modal after user uploads a file
* First pass at modal styles
* Allows user to enter a caption

@weerd @DFurnes 
Yep, [it's in](https://github.com/DoSomething/neue/blob/f0141d62de49d8c37d081ec96564930e42891d1c/scss/_base/_typography.scss#L156-L164)! :art: