Allow displaying a specific revision

Problem/Motivation

When linking to a view mode page from a non-default (non-current) revision, the view mode page will always default to the current revision, since View Mode Page doesn’t know how to interpret a revision’s base URL. Substituting a revision URL for the node URL in a view mode page pattern will not show the specific revision’s view mode.

Steps to reproduce

1. Save one or more revisions of a node.
2. If the URL for node 123 is /url/to/node, and a view mode page has the /%/custom-display-mode pattern, visit the same pattern for a different revision of the node, such as /node/123/revisions/789/view/custom-display-mode.

Proposed resolution

1. Modify the path processor to allow recognizing revision paths.
2. Add a controller to display entity revisions in specific view modes.

Remaining tasks

User interface changes

API changes

Data model changes