Switching Subscriptions Should Prompt User for Confirmation

### Expected behavior
As a user, I'm considering switching to the annual plan. I see $7 a month, billed anually and I like the sound of that. I click on it, a window pops up and I see $7 a month is $84 a year, billed once. I click confirm when prompted to change my plan, it display information for my new subscription.

### Actual behavior
I see the subscription options and I'm thinking about switching. I click on another plan and a message pops up saying my plan was successfully changed. Wait! I just wanted to know what $7 a month turns out to be annually! Too late. My card is charged immediately. I switch back and my card is chraged again. Luckily, it gets prorrated for time not used but now my billing history is hard to follow.

tl;dr: Add a confirmation prompt before switching subscriptions with additional plan information.

Start at the subscription plan-selection UI and follow the billing flow triggered when another plan is clicked. Verify how the selected plan's monthly and annual amounts are displayed and where the change is charged. Done means a confirmation prompt appears before switching, includes additional plan information, and no charge occurs when the user cancels.

Written by the indexing model from the issue text.