Remove brand identification in checkout and tweak styles

[dechov]

Fixes #22

Changes proposed in this Pull Request

• Since WooCommerce (Payments) is not a customer-facing brand, this change removes mention from the default gateway title and description.
• Also, the default description is now empty, as there is currently nothing that is in need of verbal instruction [#].
• Finally, the styles are touched up a bit, primarily to avoid wasting space.

Default:

Error state:

With description added:

(This uses a <legend> element within the <fieldset>, but can revert to <p> if that's best, in which case we should just wrap the description in wpautop.)

Testing instructions

• Go to Checkout screen with WooCommerce Payments enabled
• Verify brand, copy, and appearance changes

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• Tested on mobile (or does not apply)