Make Pageimage::size hookable per Horst's request
Add support for extended pagefile paths per issue #432. Extended pagefile paths limit the number of directories per level (in /site/assets/files/...) to under 2k per directory. To enable, set `$config->pagefileExtendedPaths = true;` in your /site/config.php file. This setting only affects pagefile paths for newly created pages from this point forward. Existing directories will remain as-is, to avoid breaking links.
Add @horst-n updates to ImageSizer class for USM sharpening per issue #445