Adjust email footer to account for self-notifications.

If a user has the self-notification option enabled, when a self-notification
email is sent, the email footer should let the user about this.

Related commit a3ac677.
When generating no-access URLs, link to wp-login.php to authenticate the
user and redirect them back to the target location.

Introduces ass_get_login_redirect_url() - to generate the login URL with the
redirect location.

Switch out all user notification and group notification links to use
ass_get_login_redirect_url().
Better phpDoc for ass_get_login_redirect_url().
On a group's notification settings page, only show end blurb if the current
forums in use are the BP legacy forums.
Set the group creator's subscription level during group save.

Detect whether we're on the 'group-settings' creation step after a group is
recorded.  If so, also save the subscription level for the group creator.

Fixes #46.
Revamp ass_clean_subject().

- Add 2nd parameter - $add_quotes - to toggle add quotes to forum topic
  subjects
- Add stripslashes() call to mirror ass_clean_content()
- Add phpDoc

Use ass_clean_subject() / ass_clean_content() for admin email notices and
welcome emails.  Fixes issues with quotes and apostrophes not being stripped.

See #45.
Remove unnecessary 'ass_group_id' hidden input field.

Use the current group object instead.
In email notice email, if group is not public, use login redirect URL.
Add a filter for the admin notice message.

See #45.