In preparation for adding automatic CORS rules creation
for direct S3 uploads, I am adding tests here and moving the
CORS rule definitions into a dedicated class so they are all
in the one place.
There is a problem with ensure_cors! as well -- if there is
already a CORS rule defined (presumably the asset one) then
we do nothing and do not apply the new rule. This means that
the S3BackupStore.ensure_cors method does nothing right now
if the assets rule is already defined, and it will mean the
same for any direct S3 upload rules I add for uppy. We need
to be able to add more rules, not just one.
This is not a problem on our hosting because we define the
rules at an infra level.
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