DEVPROD-9618 rework sharing admin config

[ybrill]

DEVPROD-9618

Description

The current implementation will be hard to use going forward: in your staging's local database there are a couple of documents that override the shared config.

• looking at the admin page you wouldn't know this
• if someone adds a field to one of those documents it'll need to be changed on everyone's staging
• you can only override something yourself by setting it manually in the database

This PR changes this. It adds a, "Environment Overrides" section to the admin page

This way it's clear what's overridden, and you can override other things too if you need to.

As before, the shared config is a noop on prod and the main staging. A change from before is that if a config document is overridden, there will be an extra round of parsing for it (into a bson.M and back to bson.Raw). The logic for getting the overridden configuration is a lot simpler now.

Testing

I deployed this to my staging and it works.