[Toaster] Image customisation implementation plan

[Barros Pena, Belen]

I've been looking into breaking down the image customisation feature into
small chunks. 

We have currently 2 features in Bugzilla that are needed for this work:

https://bugzilla.yoctoproject.org/show_bug.cgi?id=7571
https://bugzilla.yoctoproject.org/show_bug.cgi?id=7329

I have then identified 10 additional features that could be created in 6
upstream drops:

[DROP 1]
Feature 1. Changes to the Import layer page

[DROP 2]
Feature 2. Create the Image recipe details page


[DROP 3 - the biggest one]
Feature 3. Create the New custom image page
Feature 4. Implement creating custom images (involves creating new custom
image details page)
Feature 5. Implement add / remove packages functionality

Feature 6. Create the Custom images page
Feature 7. Changes to the Basic configuration page

[DROP 4]
Feature 8. Implement building new custom images

[DROP 5]
Feature 9. Implement downloading custom image recipe files

[DROP 6]
Feature 10. Implement deleting custom images

If you would rather see the feature broken down in a different way, let me
know before next Tuesday.

Thanks!

Belén