Add banner to user and developer facing AMO pages

[caitmuenster]

Editor's note: This issue is now about providing a backend mechanism to show a global notice for future site issues.

Until we have a fix for the certificate issue, let's put up a banner on all AMO pages that face users and developers that says:

.You may be experiencing issues with extensions and themes for Firefox. Please see https://discourse.mozilla.org/t/certificate-issue-causing-add-ons-to-be-disabled-or-fail-to-install/39047 for more information."

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For QA: please verify that you can update the Config (zadmin) object named site_notice and that its value is exposed in the responses of these URLs:

• https://addons-dev.allizom.org/api/v4/site/
• https://addons-dev.allizom.org/api/v4/accounts/profile/

If we set the site in read only mode, a read_only attribute should be exposed in the responses of these URLs:

• https://addons-dev.allizom.org/api/v4/site/
• https://addons-dev.allizom.org/api/v4/accounts/profile/

[AdrianoCahete]

And pin a issue here at GH, otherwise someone will need to close countless issues with the same problem.

(and don't link Discourse, cause it's returning a 429 -- Too much request).

[willdurand]

@caitmuenster I don't think this is something we can do without deploying a new version, which is risky...

[diox]

We've deployed a banner for developers specifically since approvals are currently disabled. It currently says, at upload:

Due to a certificate issue, add-on approval is currently disabled. All submissions are hold in "Awaiting approval" state. For updates, please follow the discourse thread. We apologize for the inconvenience.

[kumar303]

Once we have a backend, we could reuse this UI for showing banners: mozilla/addons-frontend#5620 (the screenshot in that PR does not show the final UI)

[muffinresearch]

Moving this to add-ons server since the first issue to solve is to add an API for this issue.

[eviljeff]

What existing API endpoint the messaging would be added to? Or is there a new API needed? Or all API endpoints are supposed to return something in the headers? @kumar303 / @willdurand what would work best for frontend?

[kumar303]

@eviljeff we would be introducing a new type of thing. I suggest an endpoint like /site/ or something to convey site status.

If a user is signed in (i.e. a cookie exists) then we always make a request to /accounts/profile on the first page load. It would be nice if that endpoint returned the data we need just so that we don't have to make an extra API request. Obviously, we'd have to make an extra API request for non-signed in users.