Add Holly Mitchell, update Robert Garcia's headshot

[hancush]

Overview

See title.

Checklist

• PR has a descriptive enough title to be useful in changelogs

Notes

Needed to remove Ridley-Thomas's memberships ending 2021, as well as the membership created for Mitchell before the corresponding scraper change came in.

LAMetroPerson.objects.get(family_name='Ridley-Thomas').memberships.filter(end_date__startswith='2021').delete()
Person.objects.get(family_name='Mitchell').memberships.filter(organization__name='Board of Directors', post__isnull=True).delete()

Testing Instructions

• Deploy to production and confirm that Holly Mitchell has a headshot and bio, that Ridley-Thomas no longer appears in the board members listing, and that Robert Garcia has an updated headshot.

Handles #681, #683