Added a reply link to the notifications/show page

- sets subject "Re: whatever" for PMs
- also removed "sender=N" from PM link (wtf! -- it wasn't doing anything
    but it looked bad.)
removed unused random crop code
Added reply link to notification email.

Note: this breaks DRY but we couldn't figure out how to put it in a
helper and make it useable by both the controller and the mailer.
Suggestions/help much appreciated!
fixed failing notifications tests

(our non-DRY code, which we just pasted into the mailer, was breaking
this in a roundabout way. this is a quick and not very nice fix. The
whole problem is resolved by @pozorvlak's recent "no subject" fix for
notifications, anyway.)
Delete unused helpers.
Move reply_link into a helper.

 - keep @reply_link as an instance var, for testability
 - always use fully-qualified URLs, to reduce code duplication
Merge pull request #193 from pozorvlak/replylink

Replylink