Disallow Gmail accounts from entering IMAP flow

Summary:
The error message from D2515 is being upgraded to also disable the
continue button so that a user is unable to sync an @gmail.com account
using the IMAP flow.

Test Plan: Tested locally.

Reviewers: bengotow

Reviewed By: bengotow

Differential Revision: https://phab.nylas.com/D2516

internal_packages/onboarding/lib/account-settings-page.cjsx

@@ -156,7 +156,10 @@ class AccountSettingsPage extends React.Component
     if providerField.type == "email" and event.target.value
       if event.target.value.endsWith('@gmail.com')
         # set a state that contains a "this is a gmail account" message
-        @setState({errorMessage: "This looks like a Gmail account. You should go back and sign in to Gmail instead."})
+        errorFields = _.uniq(@state.errorFieldNames.concat([field]))
+        @setState
+          errorMessage: "This looks like a Gmail account. You should go back and sign in to Gmail instead."
+          errorFieldNames: errorFields
         @_resize()
       else
         @setState({errorMessage: null})