#1074 Created structure and logic for storing / rendering children widgets
#1074 Refactored rendering of children widgets, added ability to pass the lists of options
#1074 Child widgets options modifications
#1074 Implemented saving of child widgets
#1074 Implemented ability to edit child widget options
#1074 Missing test modifications
#1074 UI modification for editing / removing child widget
#1074 Modified rendering of child widget options
#1074 Added ability to edit child widget
#1074 Removing widget tag, when rendering child widget
#1074 Created event for child content configure and API method
#1074 Updated server / UI logic to render widget without smart tags
#1074 HTML Agility Pack added to root module NuGet spec
#1074 dded assignment identifier, which is used for identifying child widget; Created versioning of child widgets; implemented child widgets restoring;
#1074 Changed logic for configuring / versioning / restoring child content options
#1074 Implemented checking of circular references when adding child widget; Created migration script for child contents structure;
#1074 using list of references instead of dictionary, when checking circular references; exceptions added to the resources list;
#1074 Fixed child widget options and their rendering
#1074 Fixed cloning of pages with child contents; fixed editing of HTML content child contents, blog post child content options;
#1074 Fixed preview of child widget in historical item preview and in widgets list preview;