Context and memory leak in the notifications email renderer

It just isolate the Velocity macros but that's it. Any modification made to the context will end up in the next email.

The most visible impact is when a lot of mails notification mails are generated in the same thread (for example when there is a lot of user like in myxwiki.org) you can end up with a VelocityContext instance reused for a huge number of script which is something that is not well supported: this issue was created after find a 650MB VelocityContext instance is a memory dump generated in myxwiki.org...

The solution is generally to push/pop a cloned ExecutionContext.