Widgets stick around when page type doesn't allow it

Suggested in #1147

A page with an area called `body` on a `team` page type has a widget with a grid of team members. The page type was changed to a `home` page type, which also had a `body` area but didn't have that widget in its allowed list. The widget stayed on the page, could be opened and edited, but when saved it thew an `unblessed` error.

The widget should be removed and/or throw a warning message that the type you're trying to change to won't allow all your content to stay.

Start by tracing page-type changes involving the `body` area, widget allowed lists, and the `unblessed` error described in the issue. Reproduce changing a `team` page to `home`, then verify that disallowed widgets are removed or that a clear warning prevents saving content that the new type cannot support.

Written by the indexing model from the issue text.