Import in Signup: Make isValid property in user URL submit Track lowercase

[andfinally]

Changes proposed in this Pull Request

• Properties in Track calls must be lowercase. Properties with uppercase letters cause the console error

Tracks: Event `calypso_signup_import_url_submit` will be rejected because property name `isValid` does not match /^[a-z_][a-z0-9_]*$/. Please use a compliant property name.

• Change the isValid property to isvalid.

Testing instructions

• Check out the branch, run Calypso and go to a signup flow which uses the import-url-onboarding step, for example http://calypso.localhost:3000/start/import-onboarding/ (Click on the bug icon and set the A/B test value showImportFlowInSiteTypeStep to show to see the "Already have a website?" link.)
• Open the network tab of dev tools and filter by pixel.wp.com.
• Enter an invalid URL in the input and - without blurring the input - click on the "Continue" button.
• The page should make a request to the tracking pixel http://pixel.wp.com/t.gif. As well as the flow name and the URL you entered, the request params should include a correct isvalid property.
• Enter a valid URL in the input and click "Continue" again.
• The page should make another request to the tracking pixel, passing similar params.

Related #35606

[matticbot]

Test live: https://calypso.live/?branch=fix/import-signup-flow-url-track

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[mattsherman]

Approved -- tested and observed correct property in Tracks request.