Allow in-app seats upgrades

[dabeeeenster]

For the SaaS product, customers should be able to upgrade their seats in the Scale Up plan.

We need to create an API endpoint that receives an integer with the desired number of seats and then applies it via this endpoint in Chargebee: https://apidocs.chargebee.com/docs/api/subscriptions?prod_cat_ver=2#update_subscription_for_items

It should then update the max_seats column in the Subscription model

[gagantrivedi]

What about free plans?

[dabeeeenster]

Free plans should be locked to a single seat and require at least a Start Up plan. We should not show the invite link if they are on a free plan and direct the user to the upgrade modal.