Fix template minor issues

[Chadys]

3 little changes (one for each commit):

• in checkout template, use line object everywhere
• add consistency to review file location and detail url retrieval
• make every communication template inherit from corresponding base template file. Some of them where missing the extends.

However for that last one, there is the issue of having Thanks for your interest, The {{ site }} Team in some templates, which is redundant with the base template signature.
Affected files are:

• oscar/templates/oscar/communication/emails/commtype_product_alert_confirmation_body.html
• oscar/templates/oscar/communication/emails/commtype_product_alert_confirmation_body.txt
• oscar/templates/oscar/communication/emails/commtype_product_alert_body.txt
  (Note the inconsistency between product_alert_body text and html content since the aforementioned text is only present in one of them)
  I didn't want to edit translated texts without first asking how this is done for the project.