Enhance loading of releasenotes templates

Currently only those Velocity templates are loaded into classpath which are defined by the property jira.releasenotes.templates. We now have a particular use case where we tried to provide a template only for a particular project by changing the file releasenotes-html.vm to something like this:

#if($project.name == "myproject")
  #parse("myproject-html.vm")
#else
  #parse("releasenotes-html-orig.vm")
#end

However, the file myproject-html.vm obviously cannot be loaded by Velocity since it is not in the classpath.

So for us it would be very nice to simply have all the files in the template's directory available for Velocity.