Add timeline icon for issue weights

What does this MR do?

Adds timeline icon for issue weights according to #5340 (closed).

Before	After

What are the relevant issue numbers?

Closes #7121 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

ee/app/helpers/ee/system_note_helper.rb

@@ -13,7 +13,8 @@ module SystemNoteHelper
       'issue_added_to_epic' => 'epic',
       'issue_removed_from_epic' => 'epic',
       'issue_changed_epic' => 'epic',
-      'epic_date_changed' => 'calendar'
+      'epic_date_changed' => 'calendar',
+      'weight' => 'scale'
     }.freeze
 
     override :system_note_icon_name