Remove node hard-coding

Problem/Motivation

Most of the UI (router, tabs, etc) are currently hard-coded for nodes.

src/Access/WorkflowParticipantsAccessChecker.php
54:   * @todo Don't hard-code for node.

src/Form/WorkflowParticipantsForm.php
58:    // @todo Generalize this beyond nodes.
161:    // @todo Entity access to the workflow participants entity should work

src/Routing/RouteSubscriber.php
30:    // @todo This is hard-coded for nodes.
55:   * @todo Investigate decorating the node access check instead.

src/StateTransitionValidation.php
32:   * @todo Once ported to 8.3.x version of content moderation/workflow, a

src/Plugin/Block/WorkflowParticipants.php
121:    // @todo This is hardcoded for nodes.

workflow_participants.info.yml
9:  # @todo Remove hard-coded node dependency.

workflow_participants.module
69:  // @todo Don't hard code for node.

workflow_participants.routing.yml
10:# @todo Don't hard-code for nodes.

Proposed resolution

Generalize the code to work with any moderated entities.

Remaining tasks

User interface changes

API changes

Data model changes