External templates

This is an optional (and [logical](http://chat.stackexchange.com/transcript/message/14053695#14053695)) cleanup to add to the extension builder pipeline. I can't think of any reason not to merge it into that feature branch as a package deal before [the extension build pipeline gets merged into master](https://github.com/Benjol/SE-AutoReviewComments/pull/5).

Does anybody have an objection @oliversalzburg and I may have missed?