Revert "MDL-39770 mod_chat: cleanup of the Bubble chat theme"

This reverts commit 5f06bf8554700190a3a98ef3251d6ac803aee529.
Revert "MDL-39770 mod_chat: some tweaks to make it prettier to reduce screen wastage"

This reverts commit 9035e3d24b5ee8fd8b692854bb10876d1d51e74b.
MDL-40808 remove unused DML strings