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"}