[API] Add public property to use private LanguageForge

Add static bool ServerSettingsModel.IsPrivateServer; if set to true, will use the private LanguageForge server.

Show the server URL label next to the Log in button when using the private or QA server
Try to list private projects, but continue with a warning on errors
TODO (server side): implement this API on LanguageForge (it is not documented)
Allows private server users to store their credentials and use the main interface rather than entering a "custom" URL
Addresses https://jira.sil.org/browse/LT-20567 and https://jira.sil.org/browse/LT-20571

+semver: minor

This change is

sillsdev / chorus