Move away from using a builtin template for the mail content

[rdimitrov]

We have an inline template here:

https://github.com/stacklok/minder/blob/main/internal/email/template.go

This should be really be in a configuration file somewhere, especially since it has branding and stuff on it.

[evankanderson]

We can put this content in the cmd/server/kodata and it will get packaged into the image (but can be overriden with a volume mount).