Add configuration to allow white-labeling

[jmiranda]

For now, we're just going to add a config property that will allow a system administrator to override the default title displayed in the top left next to the icon.

Admin users can already customize the header to include their own logo (which overrides the OpenBoxes text), as well background and foreground color. The white-label config property will just act as a default in case users don't want to upload a separate logo for every location.