Add the Send Message functionality to the guest list page

I went for a variable to control this. Patch supplied.

Updated patch that only assigns the page title & breadcrumbs when the form is being loaded on its own page.

Here's a patch that wraps the whole guest list with the
tag.. which opens up further possibilities.

Here's a patch that includes #964492: Change "Message audience" field to checkboxes, allow multiple select, which was necessary to make it work. The Audience selector is now a set of checkboxes, and the guestlist template now manually adds checkboxes to the status titles (class="rsvp_guestlist_answer_block_header_text"); I took a slightly hackish way of doing this without having to completely rebuild the form and had to use CSS to hide the now redundant original checkboxes, if anyone else has a better idea please let me know. Also, this works best when combined with the following other patches:

• #962964: Provide option to always display all guests
• #962988: Provide option to display all guest list statuses
• #963918: List guests in an expandable/collapsible block
• #964506: All form actions on the node pages should return to the node page
• Included: #964492: Change "Message audience" field to checkboxes, allow multiple select

Another patch... this one builds upon #4 by giving checkboxes for each individual user, so as an event coordinator you can email individual users or entire groups of users. The notes in #4 still apply, this works best when combined with several other patches that I submitted.

Yet Another Patch.. same as above only this time the Send Message icons removed from the guest list as they're now unnecessary.

One more patch.. a small tweak to add titles to the checkboxes to indicate what they are for.

Fixed a bug in the guest list display that caused emails to not be sent. Because the $guest['username'] was being appended with the ticket count, this appended string was being submitted to the code for identifying users to email and it obviously wasn't able to find a user record.

This patch fixes a bug in rsvp_function_send_message_guests() to correctly handle the $attendee => $attendee_name.