Core: new field addthis, for social media

## Ready State: NO

This pull request is related to the following JIRA tickets: 

https://imagex.atlassian.net/browse/LDC-109
https://imagex.atlassian.net/browse/LDC-112
https://imagex.atlassian.net/browse/LDC-113
https://imagex.atlassian.net/browse/LDC-111
https://imagex.atlassian.net/browse/LDC-110

## Requirements & Dependencies

* This new module requires features, addthis and imagex_addthis

## Deployment Instructions

* Merge code.

## Usage/Test Instructions

* ensure addthis field exists for page content type when this new feature is enabled.