Migrate to FreeMarker

As of Spring 4.3, the VelocityEngineFactoryBean is deprecated in favor of FreeMarker:

[INFO] src/main/java/org/genesys2/spring/config/SpringMailConfig.java: Some input files use or override a deprecated API.
[INFO] src/main/java/org/genesys2/spring/config/SpringMailConfig.java: Recompile with -Xlint:deprecation for details.

All templates that are now passed through Velocity must be converted to FreeMarker format (email templates mostly).

Add a unit test that will load both the old and the new templating engine and test that templates using FreeMarker generate same output (for same parameters) as Velocity.

Estimate?