Charge and Pay everyone

Alright, @wallawe. This is a pretty biggie. This sets the following flow:

- Someone creates a theme
- If they don't have a `stripe_account` associated with themselves, we create one for them (we store a user's `Stripe::Recipient` ID as well as their preferred method of payment on that object on our side)
- They then choose the type of payment method they prefer: credit card or bank account
- When they submit this, we create a `Stripe::Recipient` on their behalf, which allows us to pay them

Then...
- Another person buys a theme
- We create a `Stripe::Customer` for them and initiate a `Stripe::Charge` for them
- We then issue a `Stripe::Transfer` to the owner of the theme for the percentage of the theme they earn

In addition, this also:
- Adds approval capability to themes if the user is an admin
- Specifies whether a theme is not yet approved

**TODO:**

- ~~At the moment this only works for setting a credit card, not bank account~~
- ~~Also it doesn't appear to work in the view. I have to add the payment method via Stripe's dashboard~~
- We need to add ourselves as a `Stripe::Recipient` so we can transfer money to us too, as well as maybe the Semantic founders
- I think this only works for paying people in the US which we need to look into
- Add validations to the `payment_preferences#new` form
- Probably use JS to show/hide either card or bank_account form