Ensure necessary dependencies are included in `bullet_train.gemspec`

[gazayas]

Closes #714.

I also added bullet_train-super_scaffolding because we have the following lines in the Membership controller base:

https://github.com/bullet-train-co/bullet_train-core/blob/dab52ad22c932a4c7003c8747ba1c668c39f2444/bullet_train/app/models/concerns/memberships/base.rb#L16-L17

I wanted to added bullet_train-api because of this line:

https://github.com/bullet-train-co/bullet_train-core/blob/dab52ad22c932a4c7003c8747ba1c668c39f2444/bullet_train/app/controllers/concerns/account/users/controller_base.rb#L16

However, bullet_train-api already depends on bullet_train:

https://github.com/bullet-train-co/bullet_train-core/blob/main/bullet_train-api/bullet_train-api.gemspec#L37

So, trying to include bullet_train-api was giving me this error:

> bundle install
Your bundle requires gems that depend on each other, creating an infinite loop. Please remove either gem 'bullet_train' or gem 'bullet_train-api' and try again

Gemfile and bullet_train.gemspec

We're including a lot of in-house gems in the Gemfile and not the gemspec. In some instances, we're including BOTH:

https://github.com/bullet-train-co/bullet_train-core/blob/dab52ad22c932a4c7003c8747ba1c668c39f2444/bullet_train/Gemfile#L14-L22

I'm curious if we could learn from the rails repository. It looks like they don't have Gemfiles for their repositories but include everything only in their gemspecs (which I'm under the impression that's what we SHOULD do). Would be glad to undertake that task if that's the direction we should head in.

Active Record

https://github.com/rails/rails/blob/774c630068e69547fabd6bddedd62a712ae7e736/activerecord/activerecord.gemspec#L38-L39

Action View

https://github.com/rails/rails/blob/774c630068e69547fabd6bddedd62a712ae7e736/actionview/actionview.gemspec#L36-L44