Fix class name of the read marker on MessagePanel (#10745)

* Optimize mx_RoomView_myReadMarker

- Replace the compound selector with a type selector
- Nest the type selector in "mx_RoomView_myReadMarker_container"

* Conform the style rules to the naming policy

Including "container" in a class name is redundant as our naming policy tells that a string followed by an underscore (_) represents a container if it contains an element.

* Run prettier

* Replace `mx_RoomView_myReadMarker hr` with `mx_RoomView_myReadMarker`

- Remove a unit
- Hide hr's parent (mx_RoomView_myReadMarker) on Percy snapshots

* Change the class name from `mx_RoomView_myReadMarker` to `mx_MessagePanel_myReadMarker`

The read marker is specified on MessagePanel.tsx, and should be named so to avoid confusion.

- Create _MessagePanel.pcss for the style rules of MessagePanel

* Update permalinks.spec.ts

res/css/_components.pcss

@@ -68,6 +68,7 @@
 @import "./structures/_LeftPanel.pcss";
 @import "./structures/_MainSplit.pcss";
 @import "./structures/_MatrixChat.pcss";
+@import "./structures/_MessagePanel.pcss";
 @import "./structures/_NonUrgentToastContainer.pcss";
 @import "./structures/_NotificationPanel.pcss";
 @import "./structures/_QuickSettingsButton.pcss";