Likes sends notifications for post types that have no text body

### Current Behavior

We received two pull requests (#4075 and #4095) where the Likes extension is modified to only send notifications for the Post type Comment. The problem seems to be that the notification cannot render the `content` as text for any other Post type.

Although the proposed solutions are valid solutions to this problem, we need something that is future proof and extensible.

### Possible Solution

What we need to do is extend the Post type class with a property that allows us to signify whether a Post type is allowed to `notify`.

The benefit is that implementations of Post type can easily be adapted to this new feature, without sacrificing any flexibility.