[keycloak-dev] Added theme support to emails

[Stian Thorgersen]

I've just added theme support to emails. There are now FreeMarker templates for the email body, and the subject is set in message bundles. Currently there's no way to set the email theme for a Realm, but that'll be added tomorrow.

There's a fair few changes in this commit as I had to do some updates/refactoring to the existing Theme/FreeMarker code.