Make relative asset URLs absolute #93
Description
Right now we don't, so items like https://developmentseed.org/stac-map/?href=https://maxar-opendata.s3.dualstack.us-west-2.amazonaws.com/events/Belize-Wildfires-June24/ard/16/033131010322/2019-08-29/1040010051B60600.json don't download their assets correctly.