Swap Author and Assignee Selectors on issuable index view

Closes #4039

CHANGELOG

@@ -11,6 +11,8 @@ v 8.4.0 (unreleased)
   - Add CAS support (tduehr)
   - Add link to merge request on build detail page
   - Revert back upvote and downvote button to the issue and MR pages
+  - Enable "Add key" button when user fills in a proper key (Stan Hu)
+  - Swap position of Assignee and Author selector on Issueables (Zeger-Jan van de Weg)
 v 8.3.3 (unreleased)
   - Fix project transfer e-mail sending incorrect paths in e-mail notification (Stan Hu)

app/views/shared/issuable/_filter.html.haml

@@ -29,14 +29,14 @@
           = check_box_tag "check_all_issues", nil, false,
             class: "check_all_issues left"
       .issues-other-filters
-        .filter-item.inline
-          = users_select_tag(:assignee_id, selected: params[:assignee_id],
-            placeholder: 'Assignee', class: 'trigger-submit', any_user: "Any Assignee", null_user: true, first_user: true, current_user: true)
-
         .filter-item.inline
           = users_select_tag(:author_id, selected: params[:author_id],
             placeholder: 'Author', class: 'trigger-submit', any_user: "Any Author", first_user: true, current_user: true)
+        .filter-item.inline
+          = users_select_tag(:assignee_id, selected: params[:assignee_id],
+          placeholder: 'Assignee', class: 'trigger-submit', any_user: "Any Assignee", null_user: true, first_user: true, current_user: true)
+
         .filter-item.inline.milestone-filter
           = select_tag('milestone_title', projects_milestones_options,
             class: 'select2 trigger-submit', include_blank: true,