Subscription Modal UI/UX Improvements

[jpaulet]

The subscripton modal must be finished with all the org information and revamp to fit the design:

• Change: Debes actualizar tu plan para usar esta funcionalidad for Mejora tu plan to make it more generic
• Make sure the dropdown tiers corresponds with the Stripe one: 250, 500, 1000, 2500, 5000, 10000, +10000
• Make sure once a tier is selected the prices are updated and the plans that doesn't support that tier are disabled.
• When the user enters the popup, the dropdown doesn't show any members number, only a "-". In this state, each plan shows the default max_census_size of the plan (Free: 25, Esential: 250, Premium: 500, Custom: 1000). If the user selects a specific tier, all the plans updates to show the price of that tier or a disabled subscription button if the plan doesn't support that amount of voters.
• Add all the information in the text: Currently, you are subscribed to the "YourPlan" subscription. If you upgrade, we will only charge the yearly difference. In the next billing period, starting on "dd/mm/yy" you will pay for the new selected plan. ("yourPlan" and "dd/mm/yy").
• The "Contact us" buttons must go to the "contact page".
• Adapt the modal to look like the design:
  

[elboletaire]

[ ] Make sure the dropdown tiers corresponds with the Stripe one: 250, 500, 1000, 2500, 5000, 10000, +10000

The tiers dropdown gets populated from the API, which should be getting the info from stripe already /cc @emmdim