Split out & De-duplicate / 'model'ify POST.php

[wrongecho]

Johnny + I have made lots of progress on this. Still a few bits to go.

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• All components of post have now been split into their own folders in post/ (ebecdd3)
• Just need to model-ify the remaining modules (including account, asset, etc).

[o-psi]

@wrongecho what do you mean by "model-ify"-ing the modules.

[wrongecho]

@o-psi Moving the variable assignment lines of files within post/ to their own files to deduplicate them

https://github.com/itflow-org/itflow/blob/master/post/category.php

https://github.com/itflow-org/itflow/blob/master/post/category_model.php