Update Sendgrid credentials to use API key

[efatsi]

config/environments/production.rb has:

ActionMailer::Base.smtp_settings = {
  user_name:            Rails.application.credentials.sendgrid_username,
  password:             Rails.application.credentials.sendgrid_password,
  domain:               'storyboard.viget.com',
  address:              'smtp.sendgrid.net',
  port:                 587,
  authentication:       :plain,
  enable_starttls_auto: true
}

We need to update that to use an API key (which needs to be generated within Sendgrid's UI) before Jan 21