support for logo uploader

The uploader sends a `mediaUploadComplete` event to the element with `data-hqmediapath=<current path of upload>` and it works outside of vellum. This hack shouldn't be necessary.

Hopefully these help:
https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/app_manager/templates/app_manager/partials/nav_menu_media_js.html
https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/app_manager/templates/app_manager/partials/nav_menu_media_modals.html
https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/app_manager/templates/app_manager/partials/nav_menu_media.html

Sorry this is so poorly written that this isn't obvious. :'(