[PP-1] Create a validator to check if site is in maintenance mode while package manager is in use.

Have to think about whether this is should be in package manager or just Automatic updates. If PRoject Browser is installing a new module and no other code is affected it probably would not matter if you are in maintenance mode

Have to think about cron. The site would be put into maintenance without anyone knowing ahead of time

I created 📌 Should maintenance mode be suggested or enforced? Active to get feedback from Project Browser folks as to what they would like

We need to get clarity in 📌 Should maintenance mode be suggested or enforced? Active first