Added blocks to import template

[TheRealVizard]

Problem

Described in #1487

Solution

Added blocks for each section of the import template listed below:

1- confirm_import_form block for the confirm import form.
2- import_form block for the import form.
3- errors block for the import errors.
4- validation_errors block for the validation errors.
5- preview block for the preview.

[matthewhegarty]

Thanks for submitting this. I think that this should go into our forthcoming 3.0 release, just in case there are any clashes with existing implementations, so I have changed the base branch.

Feel free to add your name to AUTHORS

[matthewhegarty]

I also adjusted the indentation so we can see the difference with 'release-3-x'