Fix missing to on reassign Merge Request email to unassigned.

Factors out MR and issue email.

Before this, when you reassign to unassigned, it said on the email "reassigned to " (empty).

Now it says: "reassigned to unassigned", just like it does for issues.

This started out as a refactor but ended on a bug fix: it had already been fixed for issues at https://github.com/cirosantilli/gitlabhq/commit/39f82b7 but not for MRs because of duplication.

Found using https://github.com/UncleGene/flay-haml