[Refactor] Make `customSort()` pure

Gestion/src/views/ApplicationsView.vue

@@ -225,11 +225,11 @@ export default {
         return dateify(a) - dateify(b);
       };
 
-      let compareFunction;
-      if (column === 'applicant_birthdate') {
-        compareFunction = compareByDate;
-      } else {
-        compareFunction = compareAlphabetically;
+      const compareFunction = pickCompareFunction(column);
+
+      function pickCompareFunction (column) {
+        if (column === 'applicant_birthdate') return compareByDate;
+        return compareAlphabetically;
       }
 
       const compareFunctionReversed = function(a, b) { return - compareFunction(a, b); };