PERF: applies optimisations on chat-live pane (#20532)

- group writes when computing separators positions
- shows skeleton only on initial load
- forces date separator to be pinned when first message to prevent a pinned - not pinned - pinned sequence when loading more in past
- relies on `message.visible` property instead of checking `isElementInViewport`
- attempts to load next/prev messages earlier
- do not scroll to on fetch more
- hides `last visit` text while pinned

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

@@ -57,6 +57,7 @@ module("Discourse Chat | Component | chat-message", function (hooks) {
       onHoverMessage: () => {},
       didShowMessage: () => {},
       didHideMessage: () => {},
+      forceRendering: () => {},
     };
   }
 
@@ -75,6 +76,8 @@ module("Discourse Chat | Component | chat-message", function (hooks) {
       @onHoverMessage={{this.onHoverMessage}}
       @didShowMessage={{this.didShowMessage}}
       @didHideMessage={{this.didHideMessage}}
+      @didHideMessage={{this.didHideMessage}}
+      @forceRendering={{this.forceRendering}}
     />
   `;