Purpose

Upgrade PheWeb to use the newest version of LocusZoom (0.13.0), and modify ancillary visualizations to work with the updated d3 dependency (v5).

During testing, some minor PheWeb bugs were found and fixed as well.

Summary of changes

New toolbar button on PheWas and region plots ("Save PNG"). So far, users appear to prefer this over SVG.
New "filter genes" button; the genes track will now show as filtered to coding genes by default. This helps reduce crowding and focus the eye on most common targets.
Template strings no longer assume 0 is truthy; use new is_numeric transformation func where appropriate
Don't show "make ld reference" tooltip link if something is already the reference variant
Uses LZ 0.13, so also gets access to things like performance enhancements for TOPMED scale data, display filters, "highlight region" track, gnomAD gene constraint data (grch37 only), etc

Some tables failed to render when there were a few missing gc lambda values
Fix "highlight region" feature in "view on UCSC" variant links
Use the same version of LZ on both "phewas" and "region" pages. These are no longer separate config options.
Fix error messages when scrolling the plot area to a region with no SNPs
Removed a small amount of dead code