add migration checklist to danger (#5427)
add migration checklist to danger (#5427)

* update danger to include a migration checklist

* fix string concatenation

* any new index, not just unique

JackDanger/sentry,JackDanger/sentry,ifduyue/sentry,ifduyue/sentry,jean/sentry,jean/sentry,beeftornado/sentry,looker/sentry,JackDanger/sentry,gencer/sentry,looker/sentry,mvaled/sentry,ifduyue/sentry,ifduyue/sentry,gencer/sentry,looker/sentry,looker/sentry,beeftornado/sentry,gencer/sentry,jean/sentry,beeftornado/sentry,mvaled/sentry,gencer/sentry,looker/sentry,gencer/sentry,mvaled/sentry,mvaled/sentry,mvaled/sentry,mvaled/sentry,jean/sentry,jean/sentry,ifduyue/sentry