Consider displaying which modules were enabled/disabled after enabling/disabling on the "List Modules" page

This is a follow-up to https://github.com/backdrop/backdrop-issues/issues/3911#issuecomment-184...

If we are to add a list of modules that have been disabled, then I would like us to also add a list of modules that have been enabled. If we are to add such a list, it would need to go in the "options have been saved" message. Having the list hidden in a more/less toggle is a personal preference (so that the messages aren't too lengthy). [H/T @klonos]

Using a more/less toggle will be most easily done when #5090 is merged into core, so that issue is a blocker for this one.