Pass data to contact form

[vigliensoni]

The contact form should receive:

dataset_id
dataset_owner_email 

This data should be retrieved from the dataset page from which the public user triggered "contact dataset owner"

[vigliensoni]

For now, this was solved (a8bd579b46b179c47d9142eefe7b6638d50b6453) through passing variables by means of the template context variable c.

This way of passing data, however, is not the preferred one. They recommend using extra_vars.

See http://docs.ckan.org/en/ckan-2.7.3/theming/variables-and-functions.html