New Room List: Create new labs flag

[MidhunSureshR]

• Introduces lab flag for the new room list
• Create a base RoomListView for further development.
• Existing RoomList has been renamed to LegacyRoomList

[t3chguy]

Lets please not create a labs flag which does nothing, this creates an unnecessary support burden in the public support room due to confusion and it feeling broken from day0. Also all labs flags need documenting in labs.md

[florianduros]

I'm in favour to call the new room list RoomListView and to rename the legacy room list LegacyRoomList in the LeftPanel import.

When we will delab the new room list, we will not have to rename it from NewRoomListView to RoomListView. Also using LegacyRoomList in the import will inform contributors that this room list will be removed in X times.