Metadata mode/pages for entities

This is outlined in more detail in the entity pages spec doc, but tl;dr most entities (currently excluding dashboards and pulses) should have a view that shows metadata about the entity. E.g., a saved question would have a link to the table the question is based on, the collection it's saved in, and the fields present in the query results (along with the visibility and metadata type of the fields).

These pages will take the place of much of what the Data Reference does today.

Start by reading the entity pages spec doc referenced in the issue and reviewing the existing Data Reference. Map which entities are in scope and what metadata each page must show; done means the specified metadata pages replace the relevant Data Reference coverage.

Written by the indexing model from the issue text.