Insert load hooks for collaborator gems

[kaspth]

This means collaborator gems, particularly the bullet_train-billing(-*) ones, can hook in and help define extra functionality. Here's how that would look:

module BulletTrain::Billing::Teams::Base
  extend ActiveSupport::Concern

  included do
    has_many :billing_subscriptions, class_name: "Billing::Subscription", dependent: :destroy, foreign_key: :team_id
  end
end

ActiveSupport.on_load(:bullet_train_teams_base) { include BulletTrain::Billing::Teams::Base }

Collaborator gems must use concerns otherwise features won't be included into the class that includes ::Teams::Base. That's because regular Ruby modules don't support this kind of deep nesting but concerns have specifically added that.

Note: this is also how Rails works internally and expects you to hook into its libraries, like Active Record.

How to ship this

• Merge this PR.
• Merge Use load hooks to insert functionality bullet-train-pro/bullet_train-billing#19
• Merge Use Teams::Base load hook to insert functionality bullet-train-pro/bullet_train-billing-stripe#10
• Merge bullet_train-billing-usage PR (not needed after all!).
• Merge Use AbilitySupport load hook bullet-train-pro/bullet_train-billing-umbrella_subscriptions#2
• Open a new PR to bullet_train-core that removes all the billing related extensions in Teams::Base, then merge.
• Release new gem versions.

[kaspth]

Took me a bit to figure out locally, but I got the bullet_train-billing version loaded and working with this bullet_train version in a starter repo. I confirmed that the load hook is fired and then used by billing to insert the functionality. Note: I removed the existing billing extensions in Teams::Base to verify the new behavior.

[kaspth]

Rebased out eb622ba now that master doesn't have that anymore.

[kaspth]

Found more places we're extending these concerns with other modules. So I put in extra load hooks and I'll do a double check on the collaborator gems again.

Edit: just bullet_train-billing needed updating.

[kaspth]

Note to self: don't delete the branch yet, the starter repo still relies on it.