FEATURE: Optionally show local time for user in card (#9527)

This adds a site setting (default off) to optionally show a user's local time and timezone in their user card. For example, I live in Brisbane, and if at 3:30PM my time I were to open a user who lives in California's card I would see 22:30 (PST).

app/serializers/user_card_serializer.rb

@@ -60,7 +60,8 @@ class UserCardSerializer < BasicUserSerializer
              :primary_group_flair_url,
              :primary_group_flair_bg_color,
              :primary_group_flair_color,
-             :featured_topic
+             :featured_topic,
+             :timezone
 
   untrusted_attributes :bio_excerpt,
                        :website,
@@ -194,6 +195,14 @@ class UserCardSerializer < BasicUserSerializer
     object.user_profile.featured_topic
   end
 
+  def include_timezone?
+    SiteSetting.display_local_time_in_user_card?
+  end
+
+  def timezone
+    object.user_option.timezone
+  end
+
   def card_background_upload_url
     object.card_background_upload&.url
   end