This introduces two new settings: reviewboard_servers_config_path and

repositories_config_path. These are used to point to user-defined JSON

files for Review Board servers and repositories, respectively.
The primary reason for these files is to allow the upcoming Docker

images to point to volume mounts for these configuration files,

separating the configuration in the images (for executable paths and

other settings) from any user-specified lists of servers and

repositories.
The configuration loader will parse these files and append the results

to the reviewboard_servers and repositories lists. This means that

the repository support doesn't have to know anything about these files.

It's purely contained in the configuration loader.
Documentation has been updated to cover these new options and to provide

examples.
Unit tests have also been added to test validation and loading of these

files.

Unit tests passed.
Tested these settings in the upcoming Docker images.
Built the docs and checked that the results appeared correct.