Flatpak: Make testing elementary version of Epiphany easier

This includes a new flatpak manifest, the reason is that we need to use the io.elementary.BaseApp//juno in order to get the stylesheet and the icons.

Flatpak picks the icons from host, even if you ship the icons within the same manifest you have currently, they will not be detected in GTK Inspector. An other solution would be using the elementary BaseApp on the same manifest, but that would force every maintainer to install io.elementary.BaseApp//juno in order to run the application on Builder, which is not perfect.

This won't be needed once elementary publish their runtime as we can switch runtimes on the fly (well, only if it's compatible somehow with the GNOME one). Until that happens, this is the cleanest possible solution.

I asked Cassidy about this as it won't allow users to "theme" Epiphany on elementary anymore unless they force a different XDG_CURRENT_DESKTOP. He gave me a green light from elementary.