Refactor Email system to builder pattern

[hakito]

@effgarces Please note that the original PR #426 as well as this was just an incomplete draft. Also #425 in not yet finished in this PR. I just spent a lot of work for this refactoring, so I wanted to share it. But currently I have not time to complete the work.

So here is the rest of the description of the original PR #426:

When I was working on a feature it was quite difficult to implement it because of the current architecture of the email system. This PR holds the current state of my approach, but as it was too risky to break something else I stopped working on it. Maybe it's something for Version 3.0?

The EmailMessage is the base class for all message types and implements an interface. Some of the derived classes have a GetTemplateName method, some don't.

I thought the reason for the architecture is because of a manipulation of the EmailMessage object after it's creation. But it seems that is not the case. All code paths I found create the message and send it to phpmailer without additional modification.

I tried to replace it with a more straighforward builder pattern.

So far I replaced the first view classes with the builder pattern approach. When I came accross GuestAddedEmail wich derives from ReservationEmailMessage things got more complicated. I think this could still be resolved when using a mixture of the builder pattern and the derived class pattern here.

Todos:

• Replace all messages with the builder pattern
• Fix Forgot Password changes the language #425
• Replace EmailMessage