[Review] [NOCSL] Add url validation

[govambam]

Recreated from Constructor-io#419 for Macroscope review.

Original PR: Constructor-io#419 by @stanlp1
Status: closed (merged)

Recreated using squash merge commit - exact merged state preserved.

Original PR: Constructor-io#419

[macroscopeapp]

Add URL validation by updating utils.getCanonicalUrl to return an absolute URL and adjust tracker tests to expect canonical_url equal to 'https://localhost/'

Rewrite utils.getCanonicalUrl in helpers.js to resolve hrefs against document.location and return null when absent; add tests for canonical resolution in helpers.js; update tracker tests to compare canonical_url to a canonicalUrl constant.

📍Where to Start

Start with getCanonicalUrl in helpers.js, then review the new tests in helpers.js and the tracker expectations in tracker.js.

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Macroscope summarized 54d30d4.