Interactive preview before importing data

[rshk]

The importer should have an "interactive mode" (on by default) to preview changes that will be made to datasets, asking for user confirmation before proceeding.

We'll also need to add a CkanObject.compare(other) method to find differences between two objects.

[rshk]

💡 IDEA: Create some "generate preview" task which will generate a (HTML) preview of the changes that will be made -> store data somewhere

two options:

1. return value with custom formatter -- maybe even pointing to a separate app
2. store all the html pages (or create a pdf report?) somewhere -> point to it / a downloadable zip