Allow server URL to be pre-defined without enforcing it to be used unmodified

Allow server URL to be pre-defined without enforcing it to be used unmodified

APPLICATION_SERVER_URL can be either empty or be specified. This commit adds the new CMake option APPLICATION_SERVER_URL_ENFORCE to decide whether to enforce the
URL's unmodified use (like before, default: ON) or to allow modification by the user (new).

By default APPLICATION_SERVER_URL_ENFORCE is set to ON, to no break with the previous implementation's expectations.

If APPLICATION_SERVER_URL is empty, APPLICATION_SERVER_URL_ENFORCE will be ignored by the Account Wizard.

The previous behaviour confused me a bit with branded builds. When the URL was (usually) specified but not forced, it was simply discarded, forcing the user to
manually supply it.

Signed-off-by: Michael Schuster <michael@schuster.ms>