This makes it possible to retrieve the icon for mimetypes in javascript.
It makes no additional queries to the server to retrieve the mimetype.

* config/mimetypealiases.json added
* mimetype.js: this is where the logic resides to convert from mimetype
  to icon url
* mimetypelist.js: generated file with a list of mimetype mapping (aliases)
  and the list of icon files
* ./occ maintenance:mimetypesjs : new command for occ to gernerate
  mimetypes.js
* unit tests updated and still work
* javascript tests added
* theming support
* folder of the theme is now present in javascript (OC.theme.folder)