Email: Support use of email templates

[dospromptman]

Currently, email templates are not supported in the send emails action.

We should implement support for Lightning email templates, which at a minimum includes allowing the user to select an existing Lightning email template to use in lieu of specifying a subject and body for the email within the action.

Beyond that minimum functionality, we may want to support additional behavior such as allowing the body of the template to be modified after it is selected, creating new templates from the body of an email within the send emails action, and saving/updating existing templates after modification within the send emails action. The interface for choosing an email template could be as simple as selecting from a list or it can support recently used templates and folder organization.

Note that allowing editing of templates after selection introduces a lot of issues if the email template contains images, since the send emails action currently does not support adding images to emails.