Add mitochondrial gene and regional constraint to browser

[ch-kr]

As discussed in the project meeting yesterday, Nicole Lake has requested that we add her newly calculated mitochondrial constraint results to the browser.

Ideally, we would add her mito gene constraint to the mito gene pages as a table here:

using a similar format as the nuclear gene constraint, and mito regional constraint also to the gene pages above the ClinVar and gnomAD variant tracks:

using a similar display as nuclear regional missense constraint.

[rileyhgrant]

Related: #1339

[phildarnowsky-broad]

Pinged Nicole today to remind her about the two blurbs of help text we're waiting on.

[ch-kr]

linking this ticket: #1666. #1666 isn't urgent, but it would be great to add this fix when releasing mtDNA constraint metrics

[ch-kr]

I forgot to include this in the initial ticket, but would it be possible to create a demo browser with her results to crowd-source feedback?

[ch-kr]

help blurb text from Nicole is ready, and she is working on a blog post now