helpers.rb

- Extract a duplicated `redirect_to`
- Fix a typo: "token", not "certificate"
- Have the "Expires at" datepicker be attached to a text field, not inline
- Have both private tokens and personal access tokens verified in a
  single "authenticate_from_private_token" method, both in the
  application and API. Move relevant logic to
  `User#find_by_personal_access_token`
- Remove unnecessary constants relating to API auth. We don't need a
  separate constant for personal access tokens since the param is the
  same as for private tokens.