Application URLs: Prevent back office hosts being overwritten in a shared database setup (closes #16741)

[matthewcare]

Fix for #16741

Testing instructions:
Replicate the issue as detailed here:
#16741 (comment)

Without this change, you will receive the error

error:invalid_request
error_description:The specified 'redirect_uri' is not valid for this client application.
error_uri:https://documentation.openiddict.com/errors/ID2043

With this patch, you should be able to log in on each instance without issue.

Something to consider (as also mentioned in the issue comments), is that now we're just continually adding to the array with no way to clear it, we could end up with stale URIs in the database. This might be a concern if you change domain names, whereby the old domain is never cleared from the database.

[github-actions]

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[AndyButland]

Thanks for this @matthewcare - your fix looks like the right approach to me, though we need to consider the point you raise about old domains no longer getting cleared. I've just pushed up a couple of unit tests to verify it. I'll see about getting it reviewed tomorrow.