Remove intl from required extensions; add add date/spl as required; add others to suggest

[westonruter]

It turns out that intl isn't actually a requirement. I tried deploying changes merged in #2183 to Pantheon and upon doing so the plugin stopped initializing due to the intl extension not being installed. In fact, the extension is not required as we do a function_exists check before using it:

amp-wp/includes/class-amp-http.php

Lines 205 to 209 in 98c2545

	if ( function_exists( 'idn_to_utf8' ) ) {
	// The third parameter is set explicitly to prevent issues with newer PHP versions compiled with an old ICU version.
	// phpcs:ignore PHPCompatibility.Constants.RemovedConstants.intl_idna_variant_2003Deprecated
	$domain = idn_to_utf8( $domain, IDNA_DEFAULT, defined( 'INTL_IDNA_VARIANT_UTS46' ) ? INTL_IDNA_VARIANT_UTS46 : INTL_IDNA_VARIANT_2003 );
	}

[swissspidy]

Can we add a suggest entry for intl in that case?

[westonruter]

Is there anything that is not considered in the composer.json?! See c57348a.

[westonruter]

Cherry-picked onto 1.1 branch: ae6d1bd & d79d1f3