title: ForeignKey Inline dropdown menu in Django admin
body: <p>I have a project with the following models </p>
  
  <pre><code>class Recipe(models.Model):
      title = models.CharField("Title", max_length = 200, blank=False)
      content = RichTextUploadingField("Content")
  
  class Tag(models.Model):
      name = models.CharField("Tag", max_length = 50)
      recipe = models.ForeignKey(Recipe)
  </code></pre>
  
  <p>and the following admin.py</p>
  
  <pre><code>class TagInline (admin.StackedInline):
      model = Tag
  
  class RecipePage(admin.ModelAdmin):
      list_display = ('title')
      list_display_links = ('title')
      fields = ('title', 'content')
      list_per_page = 25
  
      inlines = (TagInline)
  </code></pre>
  
  <p>For each inline, I would like Django's admin to show a list of tag names as a drop-down menu, based on the tags that have already been entered. Of course, users must be able to add a new tag name, as a tag object, if no suitable tag exists already. I can achieve this quite easily <strong>without</strong> inline, but <strong>with</strong> it, I'm not able to. Any ideas? </p>