This PR introduces a new attribute size to the attendees shortcode in the CampTix plugin, which allows the specification of the Gravatar size for attendee images.

Fixes #1463

How to test the changes in this Pull Request:

1. In a WordCamp website, make sure you have some attendees with gravatars
2. edit the Attendees page and make sure the shortcode is as such:
   [camptix_attendees size="192"]
3. add custom CSS to make sure the attendees avatars are 192x192 px in size
4. verify in the frontend of the Attendees page that the vatars are displayed in the correct quality, not as enlarged 96px images