[Self-Hosted] Handle No More Seats on License Error

[lucasvaltl]

Is your feature request related to a problem? Please describe

Currently, errors due to licensing issues are not handled. This means that:

• If a user is blocked from signing up because there are no more seats available on a self-hosted license, the error is returned in the URL but this is unclear to the user because they just see the sign up page again.

Describe the behaviour you'd like

• The error in the URL is transformed into a notification on screen indicating that "sign up was blocked because of a lack of seats. Please contact your administrator to increase the number of seats."

Describe alternatives you've considered

• Do nothing - would result in a bad user experience.

Additional context

• Internal RFC around this topic
• Related to [Self-Hosted] Notify Self-Hosted Users when they lost features above 10 users #9971
• Related to [dashboard] Handle EE_LICENSE_REQUIRED #3918

Requires #8933

[lucasvaltl]

Internal discussion about a potential skateboard kicked off here --> potential skateboard here.