FIX: various mobile chat improvements (#22132)

- FIX: improves reactions and thread indicator touch event on mobile
These "buttons" are located inside a scroll list which makes them very specific. The general idea is to ensure these events are passive and are not bubbling to the parent.
- DEV: moves state on top level message node
- FIX: ensures popover arrow has the correct border
- FIX: makes a message expanded by default
- FIX applies the same ios scroll fix on thread and channel
- UI: better active/hover state for thread indicator
- UI: attempts to follow more closely our BEM naming scheme
- FIX: reduces bottom padding on message with thread indicator and user info hidden
- UI: add padding for first message in thread
- FIX: prevents actions backdrop to open thread
- UI: makes thread indicator resizable

plugins/chat/test/javascripts/components/chat-message-test.js

@@ -45,8 +45,6 @@ module("Discourse Chat | Component | chat-message", function (hooks) {
         )
       ),
       channel,
-      afterExpand: () => {},
-      onHoverMessage: () => {},
       messageDidEnterViewport: () => {},
       messageDidLeaveViewport: () => {},
     };