Use fully loaded node entities in routes starting with /node/{node}

[qadan]

Passing in the node ID as a string in routes is causing issues with modules that expect routes that are part of /node/{node} to have that {node} be a loaded node entity.

This can be demonstrated (and was initially observed) by enabling the Permissions by Term module, which grabs the node out of the route to determine access. Since the EmbargoesNodeEmbargoesController gets a string instead of an entity object, the node entity isn't available from the route parameters, and the call to $node->id() fails.

I'm not sure I can dig up any standards describing this as being a hard requirement, but since /node/{node} comes from the node module itself, it would make sense to align any new sub-routes with how the node module does things. It's also worth mentioning that the {node} that gets passed into the embargoes.node.embargo route is already a Drupal\node\Entity\Node object despite not having been specified, which does reinforce that this behaviour is expected.

This PR is to update the embargoes.node.embargoes route to use a node entity, to accept that node entity on the other end, and to update the EmbargoesNodeEmbargoesForm to document what's being passed in.

[qadan]

Superseded by #15