fix fallout from mailing_list_enabled -> mailing_list_mode
Fix typo Invitiation -> Invitation
Merge pull request #134 from rvoicilas/master

Fix typo Invitiation -> Invitation
invitation requests: <em> emails
tweak story submission guidelines
add Tag.active scope to deprecate tags without removing them
message view: show admin/moderator status of sender
show story downvoting usernames to moderators to detect problems

such as voting rings or users downvoting things instead of filtering
tags
reserve "moderator" and "moderators" usernames
story submission: link to previously submitted url for recent stories

if there is only one error and the story has a already_posted_story,
show a custom error dialog that links to that story.  if there are
other errors, show the standard error dialog.

closes #135