Adding PhasePage type to control page creation better

[vitorgomateus]

Having a page type for Phases would force the behavior of the pages.

• Create new page type for Phases. So that phase and services can have a phase, and no other page can.

• Enforce child page type? as done in home\models.py:subpage_types

  HomePage ⇒ NewsIndexPage, StandardPage

  StandardPage ⇒ StandardPage

  NewsIndexPage ⇒ NewsPage

• This would be more valuable if we also created a PhaseIndexPage that can only exist under the home and can only contain PhasePage, which can only contain StandardPages. With the purpose of making the Phase field only applied to PhasePage and ServicePage. I guess for now they could use the StandardPage template because it is working, it would just restrict the creation of new pages.

  Home can also have PhaseIndexPage and ServicesIndexPage but editors can’t create them, only devs, I guess.