Default chapter templates: Updated api docs and tests

Also applied minor tweaks to some wording and logic.

During review of #4750

dev/api/requests/chapters-create.json

@@ -3,6 +3,7 @@
   "name": "My fantastic new chapter",
   "description_html": "<p>This is a <strong>great new chapter</strong> that I've created via the API</p>",
   "priority": 15,
+  "default_template_id": 25,
   "tags": [
     {"name": "Category", "value": "Top Content"},
     {"name": "Rating", "value": "Highest"}

dev/api/requests/chapters-update.json

@@ -3,6 +3,7 @@
   "name": "My fantastic updated chapter",
   "description_html": "<p>This is an <strong>updated chapter</strong> that I've altered via the API</p>",
   "priority": 16,
+  "default_template_id": 2428,
   "tags": [
     {"name": "Category", "value": "Kinda Good Content"},
     {"name": "Rating", "value": "Medium"}

dev/api/responses/chapters-create.json

@@ -11,6 +11,7 @@
   "updated_by": 1,
   "owned_by": 1,
   "description_html": "<p>This is a <strong>great new chapter<\/strong> that I've created via the API<\/p>",
+  "default_template_id": 25,
   "tags": [
     {
       "name": "Category",

dev/api/responses/chapters-read.json

@@ -5,6 +5,7 @@
   "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>",
+  "default_template_id": 25,
   "priority": 3,
   "created_at": "2019-05-05T21:49:56.000000Z",
   "updated_at": "2019-09-28T11:24:23.000000Z",

dev/api/responses/chapters-update.json

@@ -11,6 +11,7 @@
   "updated_by": 1,
   "owned_by": 1,
   "description_html": "<p>This is an <strong>updated chapter<\/strong> that I've altered via the API<\/p>",
+  "default_template_id": 2428,
   "tags": [
     {
       "name": "Category",