UI should allow creating a lock for maintenance/upgrade purposes

[majormoses]

It would be nice if say when you are doing an update/upgrade of atlantis to create a lock so that anyone trying to submit pull requests will be notified of it, While you could open a PR or the api to do this it feels like it would be a more natural process to perform through the UI.

[lkysow]

This lock would be for all of Atlantis right? We could call it "maintenance mode".

[chadasapp]

I was just about to file a task on this exact issue.

Ideally, when you enable it existing plans/applies would continue until finished but no new ones would be accepted until maintenance mode is turned back off. This would allow for a graceful shutdown when you need to kill or restart the underlying pod/VM/server/coffee pot that you're running Atlantis on.

In a perfect world, this would be exposed via some RESTful API so it could be scripted :)

[atheiman]

I think this would help with our upgrade process that I described here: #320 (comment)