Default templates: Added page picker and working forms

- Adapted existing page picker to be usable elsewhere.
- Added endpoint for getting templates for entity picker.
- Added search template filter to support above.
- Updated book save handling to check/validate submitted template.
  - Allows non-visible pages to flow through the save process, if not
    being changed.
- Updated page deletes to handle removal of default usage on books.
- Tweaked wording and form styles to suit.
- Updated migration to explicity reflect default value.

app/Search/SearchRunner.php

@@ -58,7 +58,7 @@ class SearchRunner
         $entityTypesToSearch = $entityTypes;
 
         if ($entityType !== 'all') {
-            $entityTypesToSearch = $entityType;
+            $entityTypesToSearch = [$entityType];
         } elseif (isset($searchOpts->filters['type'])) {
             $entityTypesToSearch = explode('|', $searchOpts->filters['type']);
         }
@@ -469,6 +469,13 @@ class SearchRunner
         });
     }
 
+    protected function filterIsTemplate(EloquentBuilder $query, Entity $model, $input)
+    {
+        if ($model instanceof Page) {
+            $query->where('template', '=', true);
+        }
+    }
+
     protected function filterSortBy(EloquentBuilder $query, Entity $model, $input)
     {
         $functionName = Str::camel('sort_by_' . $input);