fix unreadable table view section header + improve text and layout
replace top/bottom layout guides by self.edgesForExtendedLayout = UIRectEdgeNone

a VC's top/bottom layout guides can only be safely used in
viewDidLayoutSubviews, so using the guides would force us to override
the method in numerous places. it's much simpler to just set
self.edgesForExtendedLayout = UIRectEdgeNone and install normal
constraints during loadView.

side change in this commit: EditTextController cleanup