Check for existence of IFG rastertile before generating signed URL

[drotheram]

Currently the application will generate a signed URL for any interferogram pair selected regardless of whether it has been processed and a rastertile exists.

Querying AWS before generating the signed URL every time the callback is triggered would introduce too much latency so this awareness of the existence should be handled at the application level or with a call to the API.

Once the API accurately reflects the state of processed data in the S3 bucket we can implement this check and allow/prevent the S3 request if the interferogram record does not exist.