Reviewed base64 image upload support

- Added test cases to cover.
- Altered parsing logic to be a little less reliant on regex.
- Added new iamge repo method for creating from data.
- Added extension validation and additional type support.
- Done some cleanup of common operations within PageContent.
- Added message to API docs/method to mention image usage.

For #2700 and #2631.

app/Http/Controllers/Api/PageApiController.php

@@ -60,6 +60,8 @@ class PageApiController extends ApiController
      *
      * Any HTML content provided should be kept to a single-block depth of plain HTML
      * elements to remain compatible with the BookStack front-end and editors.
+     * Any images included via base64 data URIs will be extracted and saved as gallery
+     * images against the page during upload.
      */
     public function create(Request $request)
     {