Improvements in the upload of files

Show progress bars and improve the upload of files, logos and avatars in general.
We use handlebars templates for one or two things already, it could be used here to clean up this class a bit.
See [this line](https://github.com/mconf/mconf-web/blob/branch-v2/app/assets/javascripts/app/custom_bigbluebutton_rooms/_form.js.coffee#L1) and [this file](https://github.com/mconf/mconf-web/blob/branch-v2/app/assets/javascripts/templates/custom_bigbluebutton_rooms/room_metadata.hbs) for an example.