Fix usage the default server URL (actually use it)

This also fixes setting the server url using the REACT_APP_DEFAULT_API_PATH
env var.

Fixes #185

src/LoginPage.tsx

@@ -41,7 +41,7 @@ export default function LoginPage() {
       setLoading(true);
       setFetchError(undefined);
       const etebase = await startTask((async () => {
-        return await Etebase.Account.login(username, password, serviceApiUrl);
+        return await Etebase.Account.login(username, password, serviceApiUrl ?? C.defaultServerUrl);
       }));
       dispatch(login(etebase));
     } catch (e) {

src/SignupPage.tsx

@@ -87,7 +87,7 @@ export default function SignupPage() {
         setErrors({});
       }
 
-      const serverUrl = (showAdvanced) ? server : undefined;
+      const serverUrl = (showAdvanced) ? server : C.defaultServerUrl;
       const user: Etebase.User = {
         username,
         email,