Fix bug 854992 - Make subject line of contribute email localizable

First PR against bedrock :smile:

Took @nukeador's suggestion in the bugzilla bug: rather than customize the subject line per functional area, I went with the simple and friendly, "Welcome to Mozilla!"

Couple of questions:
* The bug mentions that metrics relies on the subject line in analyzing emails--should that block merging this PR?
* Do I need to add unit tests for this change?
* I haven't extracted the new string and put it in the l10n svn repo, do I need to do that, or will bedrock's l10n crew handle it? (On Persona, @mathjazz would handle that part for us.)
* Are there cleanup opportunities I've missed within this file, simplifying the FUNCTIONAL_AREAS dict, or is it fine as it stands?
* In general, should questions like these go in the bugzilla bug instead? Not sure quite where to draw the line.