Review/Refactor the AddThis class

[doylejd]

The AddThis class was created as a helper class in the D7 version of the module. This class has been significantly down-sized with the port to D8 since the new OO design allowed for better abstraction.

@todo:

• Determine if the AddThis class is right to be converted into a Controller
• Determine if pieces of the AddThis class can be abstracted out to a better home.
• Deprecate (remove) AddThis.php if possible

[pjonckiere]

Created a pull request here: #53