Expose required fields on form

[davi-tapajos-codeminer42]

Displays an asterisk beside label on form when field is required

Issue: #1489

[marclerodrigues]

Overall it looks really good.

[pablobm]

Thank you for your contribution @davi-tapajos-codeminer42!

[sedubois]

NB: this marks fields as required even when the validation is not applied due to an if/unless:

  validates :amount, :currency, presence: true, if: ->{ stripe? }