advagg 8.x-2.3

Bug fixes (thanks for the patches and reports everyone). Also only of interest to developers, a bunch of documentation improvements & a few standards fixes.
One issue to note is that this branch is currently incompatible with Drupal 8.3.x. We'll try to have that fixed before the end of this month - see #2850263: PHP Fatal error on CacheCollector.php.

Changes since 8.x-2.0:

• Make sure not to pass an empty prefetch element
• #2853776 by NickWilde: Need disable gzip option in D8
• #2853890 by NickWilde: Flash AdvAgg Cache button on Operations page fails
• Provide human readable times in settings forms rather than timestamps
• Standards fixes as per testbot sniffer.
• #2854673 by das-peter: Avoid exception on site / module re-install
• #2838940 by Archual: Error 500 for ajax request
• #2846281 by Xen: Support Ajax GET requests
• Advagg API mainpage
• define advagg_hooks and advagg_tests API groups.
• Fix: YUI Compressor raises warnings with PHP 7.1 see https://github.com/tubalmartin/YUI-CSS-compressor-PHP-port/pull/33
• Pass $asset to minifyJsqueeze() if minifyJsmin() fails.
• #2833686 by andyrigby, nikolay.sh: Issues with ajax commands / apply bugfix from core
• fix: remove unused variable strict warning fix: only pass variables by reference.
• #2822527 by Archual: Invalid JS asset type when enabling Deferred CSS Execution
• Fix: Issue #2823808: Config read in advagg_hooks_implemented is not resiliant