Add ability to annotate LMS-hosted HTML pages

At the moment, our LMS app can only be activated over an uploaded PDF. Some instructors have asked if the LMS app could work with HTML pages created within the course site. Some more info on this use case:

> We have teachers that provide large chunks of their textbooks as HTML Canvas pages because they can add interactions with H5P, videos etc. inline and link to other Canvas-content such as assignments and discussions to better interlink the different course materials. PDF versions of course texts are also not preferred when we promote learning anywhere and anytime since PDF documents are not responsive and thus not very handy to read on mobile devices.
>
> In short: we have teachers that are slowly moving away from static PDF’s and prefer HTML-versions of their materials instead.

Note that this is a separate concept from annotating HTML pages that are hosted on the web. Instructors want students to annotate the LMS itself. Currently this is possible with the Chrome extension, but account and group provisioning isn't provided.