Better support for mtDNA

[mpoelchau]

Currently, there are 2 problems with mtDNA display in Apollo:

• the gene page link outs don't work. They're set up to search for the Name attribute at NCBI. There are no name attributes for the features on mtDNA in RefSeq (nor do there appear to be feature pages). A possible solution would be to get the Dbxref gene ID, which represents the entire mitochondrial genome.
• the gene name is in the 'gene' field for mtDNA features. We could add that to the description field (in colorbytype) so that it will show up in the display.

[mpoelchau]

In addition, we may need to separate out mtDNA into a separate Apollo instance since it uses a different genetic code, and Apollo only allows you to choose one code per instance.