Feature: LDC-146 - initial commit of node "like" functionality

## Ready State: Yes

This pull request is related to the following JIRA tickets: https://imagex.atlassian.net/browse/LDC-146

## Requirements & Dependencies

* Requires session_api module was is add in pull request https://github.com/imagex/imagex_installkit/pull/42

## Deployment Instructions

* Merge code.
* Enable the "ImageX Content Likes" Feature
* Clear caches

## Usage/Test Instructions

* Browse to a node (be aware of this issue https://imagex.atlassian.net/browse/LDC-147 when trying to flag panelizer nodes):
** Ensure you see a "Like" link
** Ensure that the like counter updates when you like the content  (ensure it can be hidden per role using the "view liked counter" permission)
** Once a node is liked ensure the "Who Liked This" tab shows (ensure it can be hidden per role using the "view who liked this node tab" permission)
* Create a panels page and:
** Ensure you can add both the "Who Liked This" block (requires node context) & "Most Liked Content" blocks & content panes