SOS-1820

I am using the upgradeProcess to remove so-welcome-theme and also setting all the sites using so-welcome-theme to use classic theme. I think upgradeProcess is a good way to do this because it will only run once.

I committed the code into all branches. For CE versions, UpgradeGroup will do nothing. For EE it will execute the script. I think this way we can update the upgradeprocess to 2_0_4 for all versions, and it won't be confusing to maintain in the future.