LPS-40533 Transactions when enabling staging - please read comments

Hey Brian,

This is about consolidating transactions in staging when enabling and disabling it as well as for the local staging background task executor.

I've moved some methods to the StagingLocalServiceImpl to have the transactions there (no other changes other than this) plus @shuyangzhou and @ehan worked together to have the transactions available properly for the background task executor.

@shuyangzhou can you please check out the changes in https://github.com/matethurzo/liferay-portal/commit/42b1e2458046e7d2307fdcdccbc3f51e4c9f794e what is basically using the utilities you have created previously?

I've also applied some UI changes, but nothing major there.

Thanks,

Máté