Fixed a regression based on the ServiceType toString implementation.

The DB code relied on the ServiceType toString matching the enum name. Moved
the displayable value to a dedicated field and reference it in the UI.

src/main/groovy/com/jdbernard/nlsongs/model/ServiceType.java

@@ -3,9 +3,9 @@ package com.jdbernard.nlsongs.model;
 public enum ServiceType {
     SUN_AM("Sunday AM"), SUN_PM("Sunday PM"), WED("Wednesday");
     
-    private String displayName;
+    private final String displayName;
 
     ServiceType(String displayName) { this.displayName = displayName; }
 
-    @Override public String toString() { return this.displayName; }
+    public String getDisplayName() { return this.displayName; }
 }