Allow "Save and Continue" for CiviMail mailing created from search results

Record hours for this issue at: http://dev.nysenate.gov/issues/4448
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Mailings create from search results do not currently allow 'save and continue'. This is particularly a problem for sites which have implemented 'review and moderation' workflow rules in CiviMail.

Unhide the 'save and continue' button for these mailings.
Use the hidden group created when the mailing is created as the 'recipient list'.

NOTE: This means that the search result set at mailing create time will be the recipient list for this mailing. Changes to the data set between mailing create and mailing send (which might add or remove participants) will NOT change the recipient list.