Exporting IMMARKUS project for publication

[rsimon]

Not a "core feature" as such, but we need to look for ways to easily turn an IMMARKUS project into an online-publishable output. (Perhaps something along the lines of a Canopy project?)

[rsimon]

Related to #337 - which is the foundational functionality. Potentially, exporting "proper" IIIF – as a package consisting of presentation manifest, annotation pages (and image, if local file!) – is all we need in IMMARKUS. After that, users can leverage other tools (like Canopy) to build web presentations.

Also relevant, as a workflow model: Using Canopy with Tropy (we could use a similar process of getting images out of IMMARKUS, and into Canopy)