Feature/constrain parent pages

[ThomasBroch]

This PR expands the tree config with an option to define, for each document type, which types of documents they can have as parents.

This feature has been implemented in three areas:

• The generation of the parent field defenition. There, the references to also take this (optional) setting into account.
• The new-page + button menu in the page tree layout.
• If the above two fail, there is also a validator that fetches the type of the selected parent document and validates it against the allowed parent types, of those are available.

[tim-vyg]

Love this feature!

[tstikvoort]

Great! There a couple of typo's.

The only thing I am missing is an empty state, when non of the pages has marked the page type in their allowedParents.

E.g. in the following configuration. You cannot create a child content page of a content page:

  allowedParents: {
    'contentPage': ['homePage'],
  }

This is a valid case, so we should support this in the UI as well

[ThomasBroch]

@tstikvoort Good catch! In the empty state, it now disables the button and shows this tooltip:

[djohalo2]

Very nice! I think we can slightly improve the empty state text and then this can be merged 👍

[djohalo2]

I think this text is just a little bit too technical for the average content editor. We can't assume the use case of why it is configured to not contain child pages, so my suggestion is to just keep it simple. Maybe something like This page cannot contain any child pages. or something similar?

[ThomasBroch]

This is one that I had been debating myself as well for longer than I would like to admit.. As, on the one hand, I don't want to get too technical. But on the other hand, I want to make it informative enough as well that people know what's up. Furthermore, I tried to avoid the word "child", as this empty state is a result of an allowed parent config. How informative do you think the tooltip will need to be?