Convert bytestring ConfigurationSetting values to Unicode before storing in the database

[leonardr]

This branch fixes bugs found during in-person testing of qa-circulation.librarysimplified.org using the SimplyE mobile app.

• Setting ConfigurationSetting.value to the bytestring foo converted it to its Python representation, b'foo', rather than converting it to the Unicode string foo.

I looked through the code for other places where we encrypt or decrypt things, and didn't find any other problems like this.

[leonardr]

yeah, unfortunately str doesn't work that way unless you specify an encoding.