Remove getTextWithHeaders

Its only user was TinderboxMailNotifier, and that's untested, undocumented, and Should Not Be Used, since tinderbox has serious security flaws, is unmaintained, and Should Not Be Used.

This also adds support for capturing header output in LogObservers, initially to support TinderboxMailNotifier, but now unused within the codebase.  Still, I can see cases where this would be useful for users.

75355ee, 1ec1dd5, and 689565e are the commits to review here.  The "Files changed" won't help much, as it contains bf65927 and 2429b31 which are from #1028.