Clarify Unsubscription Group help for seach-based CiviMail mailings and fix markup for unsubscribe page

These changes are in response to discussion on the forum regarding confusing help text for the "Unsubscription Group" field. Also fixing invalid css / markup on mailing/unsubscribe page.

http://forum.civicrm.org/index.php/topic,19962.15.html

The updated help text is:
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Search-based mailings require an "unsubscribe tracking" group.

Because this mailing is using the results of a search to provide the recipient list, you need to specify a group to track people who unsubscribe in response to this mailing.

If a recipient unsubscribes from this mailing, they will be excluded from future mailings which use the same "Unsubscription Group". If a contact in your search results has already been recorded in the selected "Unsubscription Group", they will be excluded from the recipient list.

If a recipient clicks the unsubscribe link they will see the "Unsubscription Group" title on the unsubscribe page. The selected group should have a title that corresponds to the content / type of mailing being sent.

NOTE: Contacts who are in the "Unsubscription Group" but do not match your search critera will not be included in the mailing regardless of their status in that group. This is a change from previous versions which used the term "Base Group".
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