[PULP-255] Remove backward compatibility for manifest with artifact

[dralley]

This commit removes support for manifests storing their data inside an artifact instead of using the recently introduced Manifest.data text field.

closes #1621

[dralley]

TODO: #1644 (comment)

[dralley]

@git-hyagi @gerrod3 Any idea which headers specifically he was referring to?

[git-hyagi]

I was checking the OCI spec and found this:

"In a successful response, the Content-Type header will indicate the type of the returned manifest. The Content-Type header SHOULD match what the client pushed as the manifest's Content-Type. If the manifest has a mediaType field, clients SHOULD reject unless the mediaType field's value matches the type specified by the Content-Type header."
"A successful response SHOULD contain the digest of the uploaded blob in the header Docker-Content-Digest."

and searching the sample (return web.Response(...)) provided by Lubos, I found these headers definitions (which comply with the oci spec):

pulp_container/pulp_container/app/registry.py

Lines 137 to 142 in 18dab98

	headers = {
	"Content-Type": media_type,
	"Docker-Content-Digest": digest,
	"Docker-Distribution-API-Version": "registry/2.0",
	}
	return web.Response(text=raw_text_manifest, headers=headers)

[dralley]

I'm going to treat removing this redirect as a separate issue.

[dralley]

#1974

I've assigned myself to that work. But this PR should probably go ahead and proceed as-is, w/ passing CI.

[gerrod3]

Two small fixes then lgtm!