Support template mimetypes

Unless I forgot something the only difference between template versions and normal documents is the mimetype.

Fixes #744.
Fixes #746. (still not sure if this is the best API)
> Hm, I would consider developers using the webodf.js library end-users as well?

I would too, but the developer that knows those terms is also more likely to be reading the commit messages directly rather than these summaries... or, would be able to use this summary to find the relevant commits (perhaps better aided by including a link to this PR directly).