Input WYSIWYG: Updated API to show/accept html descriptions

Also aligned books, shelves and chapters to return description content
and some relations (where not breaking API) in create/update responses
also so that information can be seen direct from that input in a
request.

API docs and tests not yet updated to match.

dev/api/responses/books-read.json

@@ -3,6 +3,7 @@
   "name": "My own book",
   "slug": "my-own-book",
   "description": "This is my own little book",
+  "description_html": "<p>This is my own <em>little</em> book</p>",
   "created_at": "2020-01-12T14:09:59.000000Z",
   "updated_at": "2020-01-12T14:11:51.000000Z",
   "created_by": {

dev/api/responses/chapters-read.json

@@ -4,6 +4,7 @@
   "slug": "content-creation",
   "name": "Content Creation",
   "description": "How to create documentation on whatever subject you need to write about.",
+  "description_html": "<p>How to create <strong>documentation</strong> on whatever subject you need to write about.</p>",
   "priority": 3,
   "created_at": "2019-05-05T21:49:56.000000Z",
   "updated_at": "2019-09-28T11:24:23.000000Z",

dev/api/responses/shelves-read.json

@@ -3,6 +3,7 @@
   "name": "My shelf",
   "slug": "my-shelf",
   "description": "This is my shelf with some books",
+  "description_html": "<p>This is my shelf with some books</p>",
   "created_by": {
     "id": 1,
     "name": "Admin",