title: mongoengine - filter by "OR" on a single field in a single query
body: <p>Here's a (really) rough example of what I'm looking for - given a document with the following schema:</p>
  
  <pre><code>from mongoengine import Document, StringField, DateTimeField
  
  class Client(Document):
      name = StringField()
      activated_on = DateTimeField(required=False)
  </code></pre>
  
  <p>How would I query it for a client that was never activated or activated before a certain point in time?</p>
  
  <p>In other words, both of the documents would show up in the results if I searched for entries without an activation date or one that occurred before 2016-07-22.</p>
  
  <pre><code>{ "name": "Bob Lawbla" }
  { "name": "Gerry Mander", "activated_on": 2016-07-01T00:00:00 }
  </code></pre>
  
  <p>I know I can do:</p>
  
  <pre><code>Client.objects(activated_on__lte=datetime.datetime(2016,7,22))
  </code></pre>
  
  <p>and</p>
  
  <pre><code>Client.objects(activated_on__exists=False)
  </code></pre>
  
  <p>but how do I combine them into one query?</p>