Moved all strings (including subject) from the data_import html emails to yml translation files, for internationalization.
Removed text email templates from the data_import process.

Nowadays, who doesn't have an html-capable email client?
Data import emails are sent in the user language. Added spanish transpation.
A query parameter with the user locale is appended to all links in emails sent by the data import features.
Removed obsolete acceptance test.