Add uploaded files to page instead of reloading it

Based on !150 (merged).

Every time the user uploaded files in app edit mode via drag and drop it caused the site to refresh in order to show the files on it. This caused all other files the user has entered information in, like description and author, to be wiped because we currently have no auto-save functionality here (#88 (closed)).

Now, when users upload files via drag and drop, Repomaker returns information about them in a JSON response which gets used by JavaScript to dynamically add the content to the page.

Fixes #150 (closed).