Deprecate `mx_RightPanel_headerButton` class (#10821)
* Replace: %s/mx_RightPanel_headerButton/mx_RoomHeader_button/g
* Conform the selectors to our naming policy: with flag (--)
- %s/mx_RoomHeader_button_highlight/mx_RoomHeader_button--highlight/g
- %s/mx_RoomHeader_button_unread/mx_RoomHeader_button--unread/g
* Update a Jest snapshot
* Move the declarations and Sass variables
- Move Sass variables to the place where they are used
