Move Luhn followups

"Luhn: Using the From Trait" now moves to just after Bracket Push (which
is our example that uses `From`). In the Luhn implementation students
will implement `From` for multiple types, so it makes sense to put it
after Bracket Push where students only implement it for one type.

"Luhn: Using a Custom Trait" moves to after Space Age. Space Age
solutions use custom traits and `From`, so pairing these exercises made
sense.

Author: whit0694

config.json

@@ -130,26 +130,6 @@
         "higher-order functions"
       ]
     },
-    {
-      "slug": "luhn-from",
-      "difficulty": 4,
-      "topics": [
-        "from trait",
-        "str to digits",
-        "iterators",
-        "higher-order functions"
-      ]
-    },
-    {
-      "slug": "luhn-trait",
-      "difficulty": 4,
-      "topics": [
-        "traits",
-        "str to digits",
-        "iterators",
-        "higher-order functions"
-      ]
-    },
     {
       "slug": "largest-series-product",
       "difficulty": 4,
@@ -283,6 +263,16 @@
         "stack or recursion"
       ]
     },
+    {
+      "slug": "luhn-from",
+      "difficulty": 4,
+      "topics": [
+        "from trait",
+        "str to digits",
+        "iterators",
+        "higher-order functions"
+      ]
+    },
     {
       "slug": "queen-attack",
       "difficulty": 4,
@@ -318,6 +308,16 @@
         "Default Trait implementation"
       ]
     },
+    {
+      "slug": "luhn-trait",
+      "difficulty": 4,
+      "topics": [
+        "Custom Trait",
+        "str to digits",
+        "iterators",
+        "higher-order functions"
+      ]
+    },
     {
       "slug": "allergies",
       "difficulty": 4,