WIP: Checkout page of paid signup flow

What does this MR do?

Provide a way for customers to pay for a bronze, silver or gold license inside GitLab.com.

Screenshot

https://gitlab.com/gitlab-org/growth/engineering/uploads/b3effa7ea9db8f25fbf1cc1fc6630e88/image.png

Requirements

• Loads the plan information for the plan they clicked on from about.gitlab.com
• Loads the correct pricing for each .com plan
• Updates the total when the user updates their seat count
• Allows the user to update their plan
• Updates the pricing when the user updates their plan
• Does not update seat count when the user updates their plan
• Does not erase any of the form information that the user has entered when the user updates plan or seats.
• When the user has selected "my company or team" to the company question on the previous screen, it updates "Your organization's GitLab subscription" title to "{{ORGANIZATION}}'s GitLab subscription" when the user enters their organization name
• When the user has selected "just me" to the company question on the previous screen, it updates "Your organization's GitLab subscription" title to "{{FULL NAMES}}'s GitLab subscription" when the user enters their organization name
• Does not display tax when tax is $0
• Allows users to revisit completed sections when they click "edit"
• Only displays edit button after the user has filled out that section (allowing them to go back to that section)
• When user clicks edit, fields are populated with previous selections
• Clicking "continue" buttons opens the next section
• Loads credit card information as Zuora iFrame to ensure PCI compliance
• Pre-fills subscription details when the user has selected "just me" to the company question on the previous screen (plan with their plan, number of users 1)
• Disables "number of users" field when the user has selected "just me" to the company question on the previous screen
• Allows user who selected "just me" to the company question on the previous screen, to enter the team flow by clicking link in "Need more users..." help text.

Closes gitlab-org/growth/engineering#52 (closed)

Since this MR is pretty large, I've broken it up into several smaller MR's:

• !21417 (merged) (API for states by country)
• !21494 (merged) (Base for the paid signup flow frontend app)
• !21500 (merged) (Checkout and progress bar components)
• !21568 (merged) (Step component)
• !21563 (merged) (Subscription details component)
• !21638 (merged) (Billing address component)
• !21640 (merged) (Payment method component)
• !21641 (merged) (Payment method component API's backend)
• !21653 (merged) (Confirm order component)
• !21736 (merged) (Confirm order component API backend)
• !21734 (merged) (Order summary component)