Persist URL fragment during client-side mobile redirection and guard against infinite redirect

[westonruter]

Summary

Fixes #5767.

This fixes an issue which can occur when a caching plugin is failing to vary the cache by query parameters. This can result in cache pollution, where a non-AMP page is cached and served for both non-AMP URLs and AMP URLs alike, and vice-versa. When a non-AMP page is served for an AMP URL, mobile redirection can end up endlessly attempting to redirect to the AMP version, even though the user is already at the AMP URL. So the fix here is merely to check that the user is not at the AMP URL already. This fixes a symptom of a misconfigured caching plugin, although the underlying cache misconfiguration issue remains to be addressed by the site owner.

This PR also fixes a related issue I discovered where client-side mobile redirection was not persisting the URL fragment. For example, if attempting to go to /#colophon then redirection would result in the user going to /?amp=1 instead of the expected /?amp=1#colophon.

Before

before2.mov

After

after.mov

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[pierlon]

Oh #5776 amends this PR, I'll take a look at that also.

[pierlon]

QA passed

The URL fragment is kept during client-side mobile redirection, as demonstrated below:

url-fragment-persistence.mp4