Revert "Rename author_username to author_id"

This reverts commit 98ab3a2a.

ee/app/assets/javascripts/analytics/productivity_analytics/store/modules/filters/getters.js

@@ -8,7 +8,7 @@ import { urlParamsToObject } from '~/lib/utils/common_utils';
  * {
  *   group_id: 'gitlab-org',
  *   project_id: 'gitlab-org/gitlab-test',
- *   author_id: 'author',
+ *   author_username: 'author',
  *   milestone_title: 'my milestone',
  *   label_name: ['my label', 'yet another label'],
  *   merged_at_after: '2019-05-09T16:20:18.393Z'
@@ -22,9 +22,7 @@ export const getCommonFilterParams = (state, getters) => {
   return {
     group_id: groupNamespace,
     project_id: projectPath,
-    // The productivity analytics endpoint accepts an "author_id", however we get "author_username" from the filtered search
-    // As a result, we need to map the two values here.
-    author_id: author_username,
+    author_username,
     milestone_title,
     label_name,
     merged_at_after: getters.mergedOnAfterDate,

ee/spec/frontend/analytics/productivity_analytics/store/modules/filters/getters_spec.js

@@ -18,7 +18,7 @@ describe('Productivity analytics filter getters', () => {
 
       const mockGetters = { mergedOnAfterDate: '2019-07-16T00:00:00.00Z' };
       const expected = {
-        author_id: 'root',
+        author_username: 'root',
         group_id: 'gitlab-org',
         label_name: ['labelxyz'],
         merged_at_after: '2019-07-16T00:00:00.00Z',