Changes user profile path to be `/user/[username]` instead of `/user/[id]`. Fixes links to the profile pages to comply with new changes to the URL format. Splits the gift mailer into two mailers: gift and gift_invite. Gift sends karma and gift_invite sends karma and links to the signup page. Sending a gift now adds the user to the user table if not already present. URL in the invite email not points to `/signup?token=[:gift_token]`. Misc styling adjustments for polish. Other bug fixes.