Seahorse is not associated with application/pgp-signature

In Tails 2.0 rc1. Reported by user, confirmed by me:

Steps to reproduce:

- Download ISO and signature to some folder

- Open folder from Places or similar
- Double click on signature file

The file opens with Gedit (the default app)

Before Jessie, the default app was the Seahorse-Nautilus verification process.

Feature Branch: bugfix/10943-seahorse-associations

Original created by @emmapeel on 10943 (Redmine)

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