Add link to IGV help forum for downloading genome

IGV-2067
Tweak downloading genome sequence dialog

IGV-2069 (item #3 & #4)
Format status message of URLDownloader better

 Now use human readable strings, e.g 12.3 MB out of 3.0 GB instead of just long integers

IGV-2069
Handle the cancelling/failure of sequence better

We would update the stored genome list file even if the sequence were cancelled, giving the false impression it was a local genome. This resolves the issue that if a genome is downloaded, with it's sequence, but the sequence is cancelled, it would still appear in the "Load Genome From Server" list.

However, a fully downloaded genome still appears in that list, which may be confusing for users.
Track genomes by Id in LoadFromServer

This means that if one has a genome defined with id hg19 (for example), the hg19 genome won't show up in Load From Server. This could be a user defined, or downloaded with sequence, or whatever
Log the DB that VariantReviewSource connects to