Improve invite signin workflow

- Include the email of the inviter and recipient in invitation emails
  to help clue people in if they were invited using the wrong address.
- Make the error messages more friendly.  Conditions are now:

    group doesn't exist: show 404.

    not invited: display page describing what might have gone wrong
    (e.g. signed in with a different email address).

    already a member: redirect to "show group"
- Immediately add the group to the session on accepting an invitation,
  to avoid being redirected to "getting started".
- Improve style of HTML invitation email with padding, un-bolded
  heading.

Other fixes:
 - stop trying to get a socket in response to "onlogin" after 10
   seconds.
 - remove some console.log's
 - remove spurious group name &ensp;