Create "unofficial" thumbs even when no thumbs channel specified

[tomcrane]

If the channel iiif-img or thumbs (or both and either in conjunction with any other channel), then we can still invoke the thumb creation #256 using a secret policy from AppSettings.

We do this so that Orchestrator can still make optimised responses for small(ish) /full/ requests.

There is a "secret" thumbnail policy - in appSettings, not in the DB. In Engine, there has been a join on any thumbnail policy provided on an asset, with this secret policy. All this means is that sizes produced and stored in S3 are a union of this policy and the explicit policy. Only the explicit policy is returned in the API, but the union of sizes is returned on a IIIF Image service in the sizes property (handled by #631).

If "thumbs" channel not explicitly requested, there is a thumbnail service there and the thumbs service application behaves no differently, but we don't assert the existence of the thumbnail service on the single asset manifest (or in the API response).

We do, however, list the sizes we made in the sizes property of the iiif-img info.json, and they obviously go into the s.json.

Acceptance Criteria

• Decide what thumbnail sizes we want to use (see comment below).
• Add new AppSetting to store these values
• Add property to strongly typed IOptions class
• Always create thumbs, if "iiif-img" and/or "thumbs" channel requested. Sizes are distinct union of specified policy and AppSetting.
• Update single-asset manifest to only render "thumbnail" service (ie if asset.HasChannel("thumbs");). Needs to be done at Manifest and Canvas level in Orchestrator.
• Do not set ThumbnailImageService on Image hydra model if !asset.HasChannel("thumbs"); in API.

[tomcrane]

Initial policyData for unofficial thumbs is THE SAME as our current default for dlc.services

[
   "!100,100", "!200,200", "!400,400", "!1024,1024"
]