Fix various layout concerns with the new room list

It had ~32px of overflow causing a horizontal scrollbar. Turns out the layout engine gets confused when hidden elements are at the end of a flexbox, so we just move them higher in the DOM (where they logically still make sense).

The remaining changes are all alignment and misc changes in that respect, and probably not obvious.

res/css/structures/_LeftPanel2.scss

@@ -131,6 +131,11 @@ $roomListMinimizedWidth: 50px;
         .mx_LeftPanel2_actualRoomListContainer {
             flex-grow: 1; // fill the available space
             overflow-y: auto;
+            width: 100%;
+            max-width: 100%;
+
+            // Create a flexbox to trick the layout engine
+            display: flex;
         }
     }
 }