Restructure admin controller

[ghost]

Admin controller has grown to be a real unordered mess of a file. I'm always frustrated finding the right place to write code for a new feature, or finding a form field in the midst of translations.

This is the norm for OpenCart, most modules have even larger admin controllers with even more spaghetti code, but it would help future development if we could section it a bit so you can easily look up what x variable is for and where to add variable x so it makes sense.

There are 5 tabs: general, subscriber sync, abandoned cart, rss, and abandoned cart status. Start at that perhaps?

[ghost]

Repeating code could be extracted into an array and read using a foreach loop. The same could be done for errors and posted fields. This would clear up a lot of repeating code while still being aligned with OpenCart standards.