Backend numerous fixes

In this merge request, there are multiple small fixes done to the codebase:

• Use lowercase netid through the application, by making the netid's lowercase in the Controllers
• Constrain the possible values of course id, year and quarter
• Change TEST to test in the netid constants of the tests.

src/main/java/nl/tudelft/ewi/tam/controllers/ControllerUtil.java

@@ -2,6 +2,8 @@ package nl.tudelft.ewi.tam.controllers;
 
 import com.fasterxml.jackson.databind.node.TextNode;
 
+import java.util.Locale;
+
 /**
  * This class provides functionality used across controllers.
  */
@@ -34,4 +36,13 @@ public final class ControllerUtil {
         return new TextNode("{\'id\': \'" + courseId + "\'}");
     }
 
+    /**
+     * Create a lowercase {@link String} from the original string.
+     *
+     * @param originalString    the string to become lowercase
+     * @return                  the lowercase string
+     */
+    /* default */ static String toLowerCase(final String originalString) {
+        return originalString.toLowerCase(Locale.ROOT);
+    }
 }