App icon picker

**Changes**
* Added the app icon color and image picker to the app settings page in the editor
* Icon color and image is now reflected on
  * App Details page
  * Discover page
  * My Profile page

**Notes**
* Need to still make SVG versions of the icons
* Icon background color options don't match the ones on the template chooser page, where i've left them as the old, colored images <- *but we're re-biulding this page soon anyway*
* Need a better regex check for the filename for the iconImage in the templates json file
* Probably should make some of this stuff into vue components
  * The picker
  * The icon when we show it

Credit to @gklo for doing a huge chunk of work in #386 to get this issue going.

@k88hudson R?
I'm not sure it's a good idea to do this in case we allow people to upload their own icons or whatever later on.

Personally I'd be in favour of keeping the `icon` property as is, and adding `iconColor` as a new property. That we we can maintain backwards compatibility, and allow for new URLs in the future