Support all mail handlers types on OnDemand

Fast debug session shows that code in DefaultMessageHandlerFactory.java gets com.atlassian.studio.theme.plugin as a plugin for com.atlassian.jira.plugins.mail.handlers.CreateOrCommentHandler. We then try to take all message handlers defined in com.atlassian.studio.theme.plugin (there's one studio specific) and match them against CreateOrCommentHandler which we're looking a handler for. Of course that fails as you can see.