As this module is no longer supported, remove it from the codebase and
update all references to it within the custom code - instead manually
wrapping nodes with the `Post` or `Talk` class, or returning it from a
Repository.
Fixes: #465
Add a `TalkCollection` which extends Laravel/Tighten's, and add a method
there for getting the events from the talks. This makes this logic more
reusable and also makes the code in the `TalkCounter` service simpler.
