Add Enum for User Actions on Waifus

Reduces the chance of user error to near 0% as they are forced to use
this enum

src/main/java/me/goudham/util/WaifuListType.java

@@ -0,0 +1,25 @@
+package me.goudham.util;
+
+import me.goudham.domain.user.User;
+import me.goudham.domain.waifu.Waifu;
+
+/**
+ * Specifies the different actions that the {@link User} can
+ * perform on a {@link Waifu}
+ *
+ */
+public enum WaifuListType {
+    CREATED("created"),
+    LIKED("like"),
+    TRASHED("trash");
+
+    private final String listType;
+
+    WaifuListType(String listType) {
+        this.listType = listType;
+    }
+
+    public String getListType() {
+        return listType;
+    }
+}