Turn JoinAssociation into an ActiveSupport::Concern

_From a recent email:_

I wanted to ask you about a design decision in arel-helpers, but I don't want to post an issue. In order to get a join association, we currently write

```ruby
include ArelHelpers::JoinAssociation
join_association(Post, :comments, Arel::OuterJoin)
```

I find this to be a strange solution, because we're supposed to include the module to get a global function, and it doesn't feel very object-oriented. Instead, wouldn't it be better if we could write this?

```ruby
Post.join_association(:comments, Arel::OuterJoin)
```