Bookreader Offline Mode for Public Domain Books

**Context**

> Is your feature request related to a problem? What is the motivation for implementing this feature?

Right now reading a book requires a persistent internet connection. It would be nice if Bookreader had the option to store books locally, at least for open access and unrestricted material. This would make the service more useful to patrons who want to read books in transit without network connection and without requiring an install of 3rd party apps.

**Proposal & Constraints**

> What is the proposed solution / implementation? Is there a precedent for this approach succeeding elsewhere?

Implementation requires discussion. Ideally the solution is one which can efficiently and securely store images within bookreader in a way which could conceivably pave a future path for secure and protected offline reading of lendable books.

> Which suggestions or requirements should be considered for how this feature needs to appear or be implemented?

**Additional Resources**

> Add any other screenshots or assets related to the feature request.

**Success Metrics**

> How will you know if this change is successful? What specific metrics are relevant and how do you expect them to change?

How many people "save" public domain books to their bookreader for offline reading.

**Stakeholders**

> @-tag stakeholders of this feature