Make current user the first user in assignee dropdown in issues detail page

Closes #3679

CHANGELOG

@@ -17,6 +17,7 @@ v 8.2.2
   - Fix: Raw private snippets access workflow
   - Prevent "413 Request entity too large" errors when pushing large files with LFS
   - Fix invalid links within projects dashboard header
+  - Make current user the first user in assignee dropdown in issues detail page (Stan Hu)
 
 v 8.2.1
   - Forcefully update builds that didn't want to update with state machine

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

@@ -9,7 +9,7 @@
         none
     .issuable-context-selectbox
       - if can?(current_user, :"admin_#{issuable.to_ability_name}", @project)
-        = users_select_tag("#{issuable.class.table_name.singularize}[assignee_id]", placeholder: 'Select assignee', class: 'custom-form-control js-select2 js-assignee', selected: issuable.assignee_id, project: @target_project, null_user: true, current_user: true)
+        = users_select_tag("#{issuable.class.table_name.singularize}[assignee_id]", placeholder: 'Select assignee', class: 'custom-form-control js-select2 js-assignee', selected: issuable.assignee_id, project: @target_project, null_user: true, current_user: true, first_user: true)
 
   %div.prepend-top-default.clearfix
     .issuable-context-title