Properly trim the file:// from file URLs in CommandExecutor

Allow loading of genome only from "load" command, instead of requiring a file as well
Fix bug in logging error, where error message could not be created properly
Move setting gs-token cookie to IGVCookieManager.

This should have the advantage that we don't necessarily need to call through to HttpUtils.openConnectionStream for this cookie to be set
Have genome get passed into FeatureDB.addFeature, rather than using GenomeManager singleton

This is so behavior is correct when changing genomes