[TC] [77366968] fix welcome dialog

* prevent welcome message from running where the div doesn't exist
* prevent the cookie from being expired prematurely

Story: https://www.pivotaltracker.com/story/show/77366968


We can reduce the DOM queries by reusing `$('#welcome_message')` –

```coffee
_welcomeMessage: (cookieName = 'user_type') ->
  element = $('#welcome_message')
  if element.length > 0
    @_triggerModal element if $.cookie(cookieName) is "new"
    @expireCookie cookieName
```