Global warning banner

[tomjeatt]

Add a permanent global warning banner, to replace the banner that was merged here:
#1550

1. Should be feature flagged so that it can be toggled per environment
2. Content should be handled as an env variable so that it can be shown without a hotfix, and so that content can be managed per environment.

This will be a permanent change, i.e. the GlobalWarningBanner will remain in app.tsx.

@danielsimao might be possible to combine it with #1549 although if getting the global status is tricky or time consuming then we'll handle them as separate tasks:

[tomjeatt]

Agreed requirements with Jay:

1. One env variable for content. If this is set, banner will be shown.
2. Optional env variable for a link. This will be generic, i.e. if set then a 'More information' text link appended to the content.
3. Placement and width styles will follow the same pattern as the welcome banner.

No design required, but will look approximately like this: