Refactor atom feeds to use common partial for entries

This involved adding a new presenter class for the index views so that the data for the index view quacks like the data for the individual country view.
Change presentation of atom summary now it's plain text.

Use simple_format to create some structure.
Remove old helper that's now unused.
Add content link to atom summaries

Govdelivery isn't pulling out the atom link and is only using the body in email alerts
Merge pull request #282 from alphagov/ta_change_description_update

Change presentation of travel advice atom summary now the field is plain text