Photo feather: added support for image source sets

Photo feather will now generate a srcset with image sizes at several
common breakpoints: 960w, 640w, 320w, plus the size requested by the
twig.
Fixed bad parsing of parent_id

Previous implementation would eroneously set parent_id to 0 or 1,
whereas it should be set to 0 or the integer ID of the parent.
Revert "Fixed bad parsing of parent_id"

This reverts commit a78ea6b6fc17a3411111320935b076973f6859da.
Revert "Photo feather: added support for image source sets"

This reverts commit 3fe06bddd6ca1ae7385498f52d74fa2e92513521.
Merge remote-tracking branch 'upstream/develop' into develop

Syncing with upstream.
Photo: srcset support; Comments: fixed parent_id

Photo feather will now generate a srcset with image sizes at several
common breakpoints: 960w, 640w, 320w, plus the size requested by the
twig. The use of a source set allows user agents to select the most
optimal image to download.

Resubmitted parent_id fix to comments.php at request of @xhezairi.
Merge pull request #230 from xenocrat/develop

Photo Feather: srcset support; Comments Module: fix for parent_id