Add ability to "lock deployments"

[sminnee]

A small group of users (i.e. those with a separate permission setting) should have the ability to lock/unlock environments for deployment.

It would be good if the locking could be set for a scheduled time (e.g. "please lock between 1pm and 3pm"). Also it would be good if the locking could be set up as a repeating weekly schedule.

Optionally, people without the permission could click "request deployment" and the person that locked the environment could approve one a case-by-case basis.

Use cases:

• A client needs to lock their UAT environment because they're using it for a big demo and doesn't want it broken
• Prevent Friday deployments without approval

[ss23]

@sminnee Is this a good case for a module? Functionality like this seems less useful for most users.