FIX: remove trailing slashes and query params on meta-tag-updater's canonical url

[renato]

This uses the same strategy from the server-side canonical URL generation to avoid inconsistencies.

[renato]

The test failures are were underlying issues being surfaced: when we use visit("route") without a leading slash, getAbsoluteURL returns something like http://localhost:3000route, which is the invalid URL exception causing these failures.

The simplest fix is just adding these leading slashes in the few places they're missing, which I think is expected.

The alternative is handling this exception before calling getAbsoluteURL, but I'm pretty sure this will never happen in a production environment, because url comes from Ember's router.

We also have a condition on getURL that relies on this leading slash to decide if it's a relative URL:

discourse/app/assets/javascripts/discourse-common/addon/lib/get-url.js

Lines 19 to 22 in 04a58a6

	// if it's a non relative URL, return it.
	if (url !== "/" && !/^\/[^\/]/.test(url)) {
	return url;
	}

[CvX]

The simplest fix is just adding these leading slashes in the few places they're missing, which I think is expected.

I submitted two PRs to plugins (see the linkbacks above) and there are two more instances of this in a single test file in core. Just go ahead and fix them here. No need to support incorrect paths 😃