Pivotal logo update

[bridgetkromhout]

Cliff notes on how to do this kind of update:

(change the reference in old events using it)
for foo in $(grep "id: pivotal" data/events/20* | cut -f 1 -d :);do sed -i '' "s/pivotal/pivotal-before-20190307/" $foo;done

(no need to retire the upcoming events' logos)
git checkout -- data/events/2019-*

(retain the historical logo)
git mv pivotal.png pivotal-before-20190307.png

(put the update in place)
cp ~/Downloads/Pivotal\ Logo.png static/img/sponsors/pivotal.png

(historical-logo yml file)
cp data/sponsors/pivotal.yml data/sponsors/pivotal-before-20190307.yml