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!
Post subjects cannot be nil
Do validation for empty and spaces-only strings
Replace blank/nil notification subjects with (no subject)
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
Added crop wrangling hints to crops form.
added validation for en_wikipedia_url
One-off rake task to clean up empty notification subjects

Also fixed admin_user task to use login_name (case sensitive) rather
than the id-like stub generated by friendlyid (always lowercase).
Merge pull request #194 from Skud/empty_subjects

*crosses fingers*
fixed linebreak causing syntax error
Merge pull request #197 from Skud/empty_subjects

fixed linebreak causing syntax error
Add validation for name in garden model with test.
make it easier to wrangle scientific names
Merge branch 'dev' of https://github.com/Growstuff/growstuff into dev
Redirect to Garden after deleting planting
Merge pull request #200 from attlebish/plantingredirect

Redirect to Garden after deleting planting