API: normalize both key and value before calculating OAuth signature

The OAuth spec indicates that the full query string should be URL
encoded. The array_walk method does not change keys so when used with a
parameter like `filter[period]=week`, the key is not properly encoded.
This fixes that by properly encoding both the key and value.
API: double-encode percent symbols when normalizing parameters
Added is_paying_customer() to easily check if a user is a WC customer:

Usage:

`
$customer = new WC_Customer();
$out      = 'Paying customer ? ';

if( $customer->is_paying_customer( 10 ) ) {
	$out .= "yes";
} else {
	$out .= "no";
}

echo $out;
`
Clear featured transients when needed Closes #5254
Optimized is_paying_customer()
Removed break line and space
Remove unnecessary OAuth code

The parameters provided to the API endpoints only contain the
parameters specified in the method signature so there’s no need to
strip out the OAuth params.
Allow query string fallback for REST API SSL auth

In some environments, the PHP_AUTH_USER/PW server vars are empty which
prevents SSL authentication from working properly. This commit allows
the use of a query string fallback (e.g.
`?consumer_key=123&consumer_secret=abc`) for providing credentials over
SSL.

Fixes #5207
Update class-wc-admin-duplicate-product.php

PHP Strict standards:  Only variables should be assigned by reference
Update class-wc-admin-duplicate-product.php

Corrected docblock for `dupe_link()`
Update class-wc-admin-duplicate-product.php

Initialize `$sql_query_sel` before loop - to avoid code validation warning.
Stay on checkout when removing coupon Closes #5274
Clean up labels and default cost to 0 for int shipping

Closes #5219
Remove post_parent so grouped simple products are also returned

Fixes #5178
woocommerce_coupon_get_discount_amount filter in coupon class

Closes #5244
More friendly/less blunt "no shipping" messages.

Closes #5276
When hierarchy is off, only show children in the cat widget. Closes
#5245
During save_meta_boxes, only save for the "main" post being saved, not nested or subsequent save_post events

fixes #5229
Prevent security error with try catch block Closes #5275
cleaned up download handler, use network_site_url instead of network_admin_url for multisite