Allow to disable redirecting after entity save

For starters, here is a cleanup of the hook_form_alter which is hard to read in the current state and does a lot of different things. The messages are split up in their own specific form_id or base_form_id alters and can be removed in 3.x since they only show a message.

The only thing remaining in the form alter is the redirect which checks on content entity interface which will prevent excuting unnecessary code on many other forms.

But this will make further changes depending on choice easier.

I ran into this issue today. I think it would be most flexible to have an option per entity type. For example we would get a checkbox on the settings for a node type "Disable redirect to edit on save" or something like that.