#6288148 Siteless checkout thank you: Add validation to site url input

Changes proposed in this Pull Request

Adds validation to the “Enter your website address:” input form.

Task: 1200479326344990-as-1200641986671515

Screenshots:

Testing instructions

1. Checkout and run this PR
2. Go to http://calypso.localhost:3000/checkout/jetpack/jetpack_scan
3. Complete siteless checkout.
4. On the “Thank you” page, enter and submit various non valid website addresses, verify you see the validation error message, “That is not a valid website URL.”
5. Enter a valid website address, (don’t submit yet) and verify the error message disappears.
6. Copy the receiptId value n the url ( paste it somewhere, you’ll need it later).
7. Change the receiptId value in the url to some random number and press enter to reload the page. (We’re doing this so that we will trigger an error when submitting the POST request.)
8. Enter a valid website url and submit the form. Verify you see the error message, “There was a problem submitting your website address, please try again.”
9. In the url, fix the receiptId with the correct id you copied earlier. Enter a valid website address, and submit the form. Verify the url was submitted successfully and you are redirected to the “Thank you completed” page.

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