Fixes for iOS QA issues, don't auto-validate 6-digit codes

[AlanSl]

I can't seem to replicate either of these two issues but these changes should have no negative side effects and would fix the only ways I could see these issues coming about:

• https://github.com/covid-alert-ny/covid-green-app/issues/618 (error not being read)
• https://github.com/covid-alert-ny/covid-green-app/issues/627
• Button border colour being misapplied on iOS
• Auto-validation of 6-digit codes potentially triggering rate limiting error on 8 digit code if user types quickly