4.4.2: hotfix for infinite loops caused by _parent

[boutell]

In response to a report from a customer, we've released a hotfix for an issue introduced in version 4.4.0. Beginning with version 4.4.0, pieces have a _parent virtual property which complements _parentUrl and provides additional information. This is a useful feature that makes it possible to render breadcrumb trails for pieces in search results. However, it also makes it possible for infinite loops to occur if a piece page is connected to a piece via a relationship field.

In version 4.4.2 of apostrophe, this was resolved by introducing the pruneParent method, which limits the properties of _parent to a tidy subset suitable for displaying breadcrumb navigation and the like. The method can be extended if you wish to have access to more information about the piece page.