Fixes for custom recipient types functionality

Thanks for working on the improved API in the 2.x branch. I'm using the hooks for custom recipient types to provide aliases for dynamically assigned groups of users.

I've found a few ways that this functionality didn't work as expected:

• First, I found that the recipient ID must be an integer. This was not clearly documented in the API file.
• Next, I found that a recipient type name with an underscore is not supported. For example, a recipient type like "mymodule_mytype" doesn't work.
• Also, I found that custom recipient types are excluded from display when viewing a list of messages or a single message. Instead I think the custom recipient name should be displayed for users.