Broken (relative) links generated by the new mailsender

The old mailsender plugin sets an ExternalServletURLFactory in the context before rendering the mail, which makes sure that all generated URLs can be used in an email, but the new mailsender doesn't do that.

To test: send a "password reset" email, notice how the first link, which is supposed to lead to the homepage of the site being accessed, doesn't work, and the source reveals that it's a relative link.