FEATURE: allows browse page in chat drawer (#27919)

This commit ensures the browse page can be loaded in the drawer and doesn’t force full page mode.

Other notable changes of this commit:
- be consistent about wrapping each full page route with "c-routes.--route-name" and each drawer container with "c-drawer-routes.--route-name"
- move browse channels into its own component, it was before in the template of the channels browse

plugins/chat/spec/system/single_thread_spec.rb

@@ -102,7 +102,7 @@ describe "Single thread in side panel", type: :system do
       it "doesn’t show back button " do
         chat_page.visit_thread(thread)
 
-        expect(page).to have_no_css(".c-routes-channel-thread .c-navbar__back-button")
+        expect(page).to have_no_css(".c-routes.--channel-thread .c-navbar__back-button")
       end
     end