Prevent auto-reloading on error pages in response to POST requests

[westonruter]

Originally in #697 (comment):

As part of #728, I just realized that this check will never do anything:

pwa-wp/wp-includes/template.php

Lines 181 to 184 in 124baab

	if ( isset( $_SERVER['REQUEST_METHOD'] ) && 'GET' !== $_SERVER['REQUEST_METHOD'] ) {
	return;
	}

This is because the offline template is always fetched via a GET request by the service worker when it is installing, so it will never not be GET. We need an alternative way of preventing the code from running on responses to non-GET requests.

Contrary to what I thought above, each reload does not cause the browser re-POST message: #728 (comment)

We need to prevent this from happening:

infinite-reload-on-post-submission-failure.mov

We need to export the request method from the service worker fetch handler to the JS in the wp_service_worker_offline_page_reload() so that it can decide whether or not to try reloading.

[codecov-commenter]

Codecov Report

Merging #763 (54ed60b) into develop (f93356d) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop     #763      +/-   ##
=============================================
- Coverage      19.03%   18.96%   -0.07%     
  Complexity       346      346              
=============================================
  Files             57       57              
  Lines           2322     2320       -2     
=============================================
- Hits             442      440       -2     
  Misses          1880     1880              

Flag	Coverage Δ
php	18.96% <ø> (-0.07%)	⬇️
unit	18.96% <ø> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wp-includes/template.php	7.69% <ø> (-3.42%)	⬇️

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[thelovekesh]

Changes LGTM 👍🏼

[pooja-muchandikar]

QA Passed ✅

The changes are working as expected. Automatic page reload is not happening if user is making the POST requests in offline mode.