Allow for presentation of LTI tools without a Course Navigation placement

[ssciolla]

Right now, we filter out tools that don't have a Tab in the course context with a matching Canvas ID. This results in tools that don't have Course Navigation as a placement getting filtered out. Ideally, these would still be shown but the Add/Remove button on the card would be hidden (and maybe some other kind of label would be present).

I think we could change the filtering to not exclude these, change the LtiToolWithNavSerializer such that it allows for a none value and is more specific about the type of navigation enabled, and then make the button rendering be conditional in the frontend. I'm imagining the serializer would end up having something like this.

# For when it has a Course Navigation placement
{
  course_navigation: {
    enabled: true (or false)
  }
}

# For when it doesn't
{
  course_navigation: null
}