Added webp upload and thumbnailing support

Bug fixes and consolidation of various thumbnail and resize functionality
Changed resize/rotate extensions to use replace image event
Added content-disposition header to image responses to provide a human-friendly filename when saving
Added more bulk thumbnail regeneration tools
Tweaks to bulk actions to correct totals when batching items

core/exceptions.php

@@ -35,3 +35,10 @@ class ImageDoesNotExist extends SCoreException
 class InvalidInput extends SCoreException
 {
 }
+
+/*
+ * This is used by the image resizing code when there is not enough memory to perform a resize.
+ */
+class InsufficientMemoryException extends SCoreException
+{
+}