Add cost to event form

[RJV333]

What is the goal of this PR and why is this important?

1. Moves Flipper Flag for stripe_payments to :development so it isn't enabled on production ( if I'm understanding that correctly)
2. Use Flipper Flag to enable a new 'Event Cost' field on the Event form
3. Adds virtual attributes 'cost' so that the form's dollar amount input is cast from a float to an integer for the cost_cents field on events.
4. Update Event Show page to include 'Cost:' when Event has a price
5. Update existing specs

[maebeale]

Perfect! Looks great, @RJV333 !

[maebeale]

Love the use of the virtual attribute