FEATURE: Conditionally change back button route for thread (#22129)

When clicking back from a thread, we want to either go back to the
channel if the thread was opened from an indicator, or to the thread
list if we opened it from there. Since ember doesn't give a nice way
to get the previous route, we need to store this ourselves. We only
do this on mobile, on desktop we just follow existing behaviour.

Also implements a chat router history.

---------

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

plugins/chat/spec/system/page_objects/chat/chat_side_panel.rb

@@ -3,6 +3,10 @@
 module PageObjects
   module Pages
     class ChatSidePanel < PageObjects::Pages::Base
+      def open?
+        has_css?(".chat-side-panel")
+      end
+
       def has_open_thread?(thread = nil)
         if thread
           has_css?(".chat-side-panel .chat-thread[data-id='#{thread.id}']")

plugins/chat/spec/system/page_objects/chat/chat_thread.rb

@@ -58,17 +58,17 @@ module PageObjects
         header.find(".chat-thread__close").click
       end
 
-      def back_to_list
-        header.find(".chat-thread__back-to-list").click
+      def back_to_previous_route
+        header.find(".chat-thread__back-to-previous-route").click
       end
 
       def has_no_unread_list_indicator?
-        has_no_css?(".chat-thread__back-to-list .chat-thread-header-unread-indicator")
+        has_no_css?(".chat-thread__back-to-previous-route .chat-thread-header-unread-indicator")
       end
 
       def has_unread_list_indicator?(count:)
         has_css?(
-          ".chat-thread__back-to-list .chat-thread-header-unread-indicator  .chat-thread-header-unread-indicator__number",
+          ".chat-thread__back-to-previous-route .chat-thread-header-unread-indicator  .chat-thread-header-unread-indicator__number",
           text: count.to_s,
         )
       end

plugins/chat/spec/system/page_objects/chat/components/thread_list.rb

@@ -15,6 +15,10 @@ module PageObjects
           component.has_no_css?(".spinner")
         end
 
+        def open_thread(thread)
+          item_by_id(thread.id).click
+        end
+
         def has_thread?(thread)
           item_by_id(thread.id)
         end