[Feature]: Make email templates customizable

[ikreymer]

What change would you like to see?

Support for e-mail templates was added with #1375, however, they are all loaded from the email-templates directory into a ConfigMap. There is no easy way for a user to override one of the templates without modifying the helm chart itself.

Context

Like other settings, users should be able to override each of the e-mail templates with a custom value passed into the Helm chart. This value should be loadable from the any file.

Note that although Helm does not allow the chart to access any files outside the chart itself for security reasons, Helm provides the --set-file value=<filename> option which can be used to set a particular value to the contents of a file.