Currently, storage.js and main.js have many open-coded calls to
getCurrentValue for "rustdoc-" values, but these are settings and
should be handled by getSettingValue.
So make getSettingValue part of storage.js (where everyone can call
it) and use it everywhere.
No functional change yet. We are going to make getSettingValue do
something more sophisticated in a moment.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>