🟢 feat: Unleash maintenance mode

[sjaanus]

Describe the feature request

Allow unleash to enter maintenance mode.

The needs:

• Lock down all changes to unleash during maintenance mode
• Unleash is still accessible in read-only mode
• Needs to have a clear message in the UI
• Client API still works as before.
  • can still fetch toggles
  • can still post metrics

Solution suggestions

Iteration 1:

• First iteration could just be an env-flag
• locking down POST,PUT, DELETE for /api/admin
• "override permsissions" when fething users.
• Some kind of a middleware handling this

Iteration 2:

• Evaluate if certain users roles should be able to bypass this
• Time schedule for when it enters and leaves maintenance mode.

[ivarconr]

This feature will go in to beta in v4.20

[ivarconr]

Will go in to GA in v4.22