Default response and path conditions absent in 2.x

Reviewing the code changes between 1.x and 2.x, I agree that the conditions for response code and path were not included in the 2.x version, and I couldn't find any statements justifying removing them.

The maintainers' rationale for excluding 403 and 404 pages by default still seems to hold true ( #3020923: Why should GTM not be on 404 pages by default? → ), and a relatively recent change to the default path exclusions seems to indicate the maintainers still think those make sense ( #3249217: Add Layout Builder path to default path exclusion → ).

The attached patch provides the same default response codes (403, 404) and path exclusions (known admin routes) as in 1.x.

The previous patch omitted an important check to only set the default page path for new forms. This includes it.

Some of the NightwatchJS tests were failing because they were visiting a 404, which, with the new default, would not include the Google Tag Manager. I've updated those tests to (a) demonstrate that the Google Tag Manager is not present by default on a 404 and that it is present on a valid route.

Will need to retest after moving to Gitlabci -- I think the tests are still failing here.