Add user agent exclusion list for whether or not to do mobile redirection (to exempt tablets from redirection)

[westonruter]

Feature description

Some users may only want to redirect mobile visitors to the AMP version, as the "mobile redirection" implies. Nevertheless, the the user agent list currently used for redirection will match tablet devices as well:

amp-wp/src/MobileRedirection.php

Lines 281 to 289 in 0aecc3e

	$default_user_agents = [
	'Mobile',
	'Android',
	'Silk/',
	'Kindle',
	'BlackBerry',
	'Opera Mini',
	'Opera Mobi',
	];

In particular, the user agent for "Mobile" will end up matching Safari on iPad, for example:

Mozilla/5.0 (iPad; CPU OS 12_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148

And “Android” will match any Android tablet.

This being the case, maybe we should introduce an exclusion list of user agents in addition to the current inclusion list of user agents. Redirection would only be performed if the user agent does not match the exclusion list and it does match the inclusion list.

Currently the inclusion list is filtered by amp_mobile_user_agents.

The exclusion list could be filtered via amp_non_mobile_user_agents.

It seems necessary to introduce an exclusion list rather than to make the agents in the inclusion list more restrictive, as it is very difficult to construct a regular expression that matches mobile but not tablet at the same time. (If I'm wrong, please enlighten me!)

What then remains to be seen is whether tablets should redirect by default or not. In other words, should the exclusion list in amp_non_mobile_user_agents be initially populated with common tablet user agents? Or should it be empty, requiring users to add the PHP filter to prevent certain user agents from redirecting.

Originally discussed in a support topic.

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

Relates to #5293 in how it may need to be added to the settings screen.

[westonruter]

Removing from milestone since nobody has requested this other than one user in a support topic.

[westonruter]

This has only been asked for once.