Remove hardcoded URLs

The API URL will be defined in the (to-be-written) configuration file.
Make getter/setter for API URL
Remove getter/setter for API dev mode flag

No longer needed; this can be easily set in the (forthcoming)
configuration file.
Replace DataURL#getApiUrl with DataManager#getApiUrl

The new DataManager method supersedes the DataURL method.
Move definition of dev mode base URL to DataURL

This simplifies Constants, slightly.
Set up client configuration JSON file and JSO class

The file is **war/config.json**. ConfigurationJSO maps it.
Enable loading of configuration values from file

The values are stored in DataManager#config.
Update call to get API URL

It's now DataManager#config#getApiUrl().
Replace select Constants with values from config file

Currently, VERSION, RELEASE_DATE and WMT_TITLE.