Configurable form submission email templates in the UI

[tanc]

This feature request is an extension to #229 and would allow for per site/branch configuration of the form submission email template. This could take the shape of a single textarea which overrides the default (or customised) email template found in ./emails/templates/form-submission.hbs. Maybe behind a checkbox with a label: "Custom email template" or similar.

There would need to be a section in the docs on this, explaining or giving an example of how to display specific form fields. For example matching a form field named message with the template by printing {{formData.message}} (if that is how it works, I haven't tested).

This way a user could customise their form submission template from the UI and get a more tailored experience.

[tanc]

Also a field for the subject line makes sense I think.