fix https://github.com/sagemathinc/smc/issues/325 -- "Some actions on a cell sagews create an additional blank cell" -- by not making new cells except on execute.
fix https://github.com/sagemathinc/smc/issues/325 -- "Some actions on a cell sagews create an additional blank cell" -- by not making new cells except on execute.

sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc