Decompose large blocks in FlaskView

I found this project while writing a completely parallel class, and given that the work's already done, I ran with it.  One issue I had was wanting to subclass FlaskView to override some pieces of behavior, but because of method body length, much copy/paste was required to make the minor behavior changes I was after.  This pull request involves no behavior changes, but just pulls a few things out into their own space on FlaskView, so clients can override them without affecting anything else.