Mail sender plugin may use the wrong Velocity context

It's getting it from XWikiContext without checking if there is actually one while proper VelocityContext should come from VelocityManager which make sure you get one and also that it's in sync with ScriptContext.

This makes impossible to use velocity tools in a mail template executed from some background thread which most of the time don't really care about Velocity itself and often have a XWikiContext with no VelocityContext at all in it.