Handle creating candidates from email templates

[grosscol]

Email or message templates (see knowledge base) will reference a display template that they are designed to incorporate. When generating candidates from email templates, look up the display template graph and incorporate the attributes from the referenced display template.

When construction candidate:

• Use the display template as the ancestor template
• Merge intervention properties from email template and display template

[grosscol]

There will need to be a method of referring to the email template that was used to create the candidate. However, ancestor template is currently expected to be a display template by pictoralist. Referencing any display templates needed to be generated and the email template will need to be sorted out. Likely, this will mean creating a new term for the ancestor display templates and refactoring the ancestor template to point at the encapsulating email or message template.

[grosscol]

Obviated by #9