Update browser release HT for v4.1.1

[ch-kr]

PR adds RMC coverage stats HT and updates globals for browser release HT and adds two new fields, low_coverage and no_color. PR test run is in gs://regional_missense_constraint/notebooks/kc_update_browser_ht.ipynb, though I didn't rerun the browser table creation after fixing the no_color annotation

Other minor changes:

• Updates path to gene models HT and adds various TODOs to update resources
• Streamlines get_constraint_transcripts

[Ruchit10]

I didn't do a test run of this but the code LGTM. The CI checks should work once this is merged in addition to the other PR