Add modal to let users know they can't buy storage or minutes on GitLab

What does this MR do and why?

This MR introduces the new LimitedAccessModal that we will show when the limited_access_modal FF is ON. We plan a second iteration when we will also call an API in this case and only show it when it is a reseller OR ramp subscription.

Tech plan

Step	MR
Introduce the modal	Add modal to let users know they can't buy stor... (!129790 - merged)
Wire up the logic with API	Draft: Create add seats modal for subscriptions... (!120047 - closed)

Screenshots or screen recordings

With FF OFF	With FF ON
Screen_Recording_2023-08-21_at_13.23.48	Screen_Recording_2023-08-21_at_13.22.12

How to set up and validate locally

1. Create a subscription

2. Link it to a GitLab namespace either in the CDot UI (subscriptions/SUBSCRIPTION_NAME/edit?transaction=change_gitlab_group) or via the rails console

3. In GitLab, navigate to /groups/GROUP_NAME/-/usage_quotas#seats-quota-tab

For FF off

1. Verify that Add seats button appears as it should
2. Verify that Add seats button redirects to CustomersDot

For FF on

1. Run rails c and then Feature.enable(:limited_access_modal)
2. Verify that Add seats button appears as usual
3. Verify that clicking the button displays the modal

• By default, it will show the modal text for a reseller subscription because we're temporarily stubbing the limitedAccessReason
• To see the ramp subscription text, you can modify this line to return RAMP_SUBSCRIPTION

1. Verify that closing out of the modal functions properly

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #409750 (closed)