Node Keep

Adds two base fields to all nodes.

Node keeper: prevents people who don't have the 'bypass node access' permission
from deleting nodes where that checkbox is enabled.

Alias keeper: prevents people who don't have the 'administer node_keep per node' permission
from changing the node's alias if the Alias keeper checkbox is checked. Is only
installed if the 'pathauto' module exists.

Use-case: you've set up a node as an overview, and other nodes have their alias
set to be [this-node-alias]/their-own-alias. With this module you can protect
the alias you set, as well as make sure the node doesn't get deleted.

@todo: Prevent the alias config entity from being edited/deleted if it's
attached to a node with Alias Keep checked. Currently it only prevents changes
in the node edit form, so users who can manage aliases through the overview
can still change/delete it.