check for recently submitted urls before showing the story submission page

this is already done after submitting, which turns it into an
upvote, but now it will catch it when hitting the page from the
bookmarklet so the user doesn't have to fill out tags and stuff
first just to find out it's already submitted.

since this requires no confirmation, don't count it as an upvote
automatically, just redirect to the story page
move tag css class building to Tag model

add a tag_is_media class to tags with that type, use that for
matching in application.css instead
tweak invitation e-mail subject and text

"welcome to <site>" makes it sound like you already got signed up,
say "you have been invited to <site>" instead
minor css tweaks

- ditch vendor prefixes for css 3 things, fuck old browsers
- add a shadow around user profile avatar
- whitespace