UX: composer use BEM + small tweaks to enabled/disabled styling (#21246)

- using BEM notation
- making animation linear instead of default ease
- small tweaks to composer state (disabled/send-disabled/send-enabled)
- fixing bug with disabled composer on mobile

plugins/chat/spec/system/chat_composer_spec.rb

@@ -321,7 +321,7 @@ RSpec.describe "Chat composer", type: :system, js: true do
       chat.visit_channel(channel_1)
       find("body").send_keys("1")
 
-      expect(page).to have_css(".chat-composer--send-disabled")
+      expect(page).to have_css(".chat-composer.is-send-disabled")
     end
   end
 end

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

@@ -18,7 +18,7 @@ module PageObjects
       end
 
       def click_send_message
-        find(".chat-composer--send-enabled .chat-composer__send-btn").click
+        find(".chat-composer.is-send-enabled .chat-composer__send-btn").click
       end
 
       def message_by_id_selector(id)

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

@@ -50,7 +50,7 @@ module PageObjects
 
       def click_send_message(id)
         find(thread_selector_by_id(id)).find(
-          ".chat-composer--send-enabled .chat-composer__send-btn",
+          ".chat-composer.is-send-enabled .chat-composer__send-btn",
         ).click
       end