Improve JIRA header image background

Rather than having a flat colour behind the JBoss Community logo in our JIRA header it would be nice to have a more interesting background like the Hibernate JIRA server has:

https://hibernate.onjira.com/secure/Dashboard.jspa
https://www.jboss.org/dms/hibernate/images/jira/jiraheader_hibernate.png

We should still keep the vertical height the same so it doesn't take up too much space.