Add email templates and email attendees about their allergy or accommodation

[CdrMarks]

This PR attempts to add two new fields to the list of email templates in CampTix. While the additional textareas have been added and I'm able to save content and see the entered value appear again, I'm unable to correctly setup the default value for these new fields. This is why this PR is still marked a Draft.

Attempts to Fix #764

Props

Screenshots

The new textareas

How to test the changes in this Pull Request:

1. Apply the changes in my branch to production
2. Create a new WordCamp and its site.
3. Browse to the new site's Tickets > Setup > E-mail Templates (/wp-admin/edit.php?post_type=tix_ticket&page=camptix_options&tix_section=email-templates)
4. See two new fields (screenshot above) that are empty. I was under the impression that the function custom_email_template_default_values( $options ) in public_html/wp-content/mu-plugins/camptix-tweaks/addons/accommodations.php and public_html/wp-content/mu-plugins/camptix-tweaks/addons/allergy.php would set these defaults.
5. Enter some text in the two blank textareas and click Save Changes.
6. See the same text shown in the textareas.
7. Click the Reset Default button.
8. Same as in step 4 above (the fields are empty, but I was expecting to see default values.)