[Enhancement]: Add Cancel Subscription Button in "Billing" Section

### Problem Statement

Currently we get a lot of repeat tickets related to cancel subscription.
We should provide a button, which opens a dialog which tells the users, the site will be deleted inorder to stop the subscription & lets the user to choose the site (maybe server) to drop.

### User Story (Added by Neha)

As a user who wants to cancel subscription on Frappe Cloud, I should
- Easily find the cancel subscription option in the billing section (shouldn't feel like a dark pattern)
- Get clear instructions about how to cancel my subscription including
- Steps to delete the sites, benches and servers in my account
- Steps to clear my pending invoice amount
- What happens to unused prepaid credits
- Once I cancel subscription, I should get clear confirmation

#### Wireframe

Something like this: