[Toaster] Design - New custom image and Custom image details pages

[Barros Pena, Belen]

I've opened Bugzilla features to track the front end implementation of 2
of the new pages we need for image customisation:

1. New custom image, where you start the process of creating a new image

https://bugzilla.yoctoproject.org/show_bug.cgi?id=8081

2. Custom image details, where you add and remove packages to / from
custom images

https://bugzilla.yoctoproject.org/show_bug.cgi?id=8082

Design documentation attached to the Bugzilla entries as usual. Any
comments, please let me know.

Thanks,

Belén