Ensure that all lines in the pad are marked with class `ace-line`.
Ensure that all lines in the pad are marked with class `ace-line`.

Without this change, lines that haven't ever been edited will have either
an empty class or, in the case of list start lines, a class that begins
with a space (because the `ace-line` before the space never got added).

cloudfoundry-community/etherpad-lite-cf,storytouch/etherpad-lite,joequant/etherpad-lite,lpagliari/etherpad-lite,storytouch/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,aptivate/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,lpagliari/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,joequant/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,lpagliari/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,University-of-Potsdam-MM/etherpad-lite,lpagliari/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite