Abstract email templates into individual classes

[dparker1005]

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Changes proposed in this Pull Request:

Creating an abstract class for email templates will allow us to avoid duplicate code that is common across every email template. Separating each template into a separate class will then make it easier to modify existing emails and add additional email templates down the road.

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[kimcoleman]

In looking at this deeper now it seems dev does have "duplicate" variables like login_link, orders_link, and levels_link. They seem to be just populated in the PHP, not shown as "variables" you can use. Probably in an effort to reduce confusion. Maybe we had to keep these because of old merged in Add On, like CONPD?

IF we keep both the _link and _url versions just to make sure the populate, let's only show the _url versions, which seem to be what we showed in our variable reference from.