Api: Replace database with file cache

Turns out reading/writing from a sqlite database is pretty darn slow.
Reading/writing to/from a file turns out to be much faster. This data
also shows up under "Cache" in the App Info settings page so users can
clear that out independently from the core app data.

Note: The DB class is still around because we will eventually use it for
storing offline guides and converting it from its existing state should
be pretty straightforward.