move shared functions from rsmanage

[bnmnetp]

@bjones1

After not coming up with anywhere else to put these besides starting a whole different package I think this makes the most sense. These functions are used by the rsmanage command as well as the web interface for building and managing books.

If you have a different suggestion I'm happy to discuss.... I considered the following:

• code shared by bookserver, runestone server, and other new servers should go in runestone
• code shared by bookserver, runestone server, and other new servers should go a new runestone_common that we package separately from runestone components
• code shared by bookserver, runestone server, and other new servers should go in an rsmanage package

[bjones1]

This makes sense to me. However, I'd suggest adding comments / docs explaining that this code is shared across servers / repos.

Is the longer-term goal to move rsmanage to the BookServer? I assume it's a prereq to instructions being able to just pip install bookserver and run a SQLite-based server.