[realbooru] fix extraction (#6543)

- extract data from HTML pages since API is no longer usable
- move code into its own separate 'realbooru' module

docs/supportedsites.md

@@ -769,6 +769,12 @@ Consider all listed sites to potentially be NSFW.
     <td>Comic Issues, Comics</td>
     <td></td>
 </tr>
+<tr>
+    <td>Realbooru</td>
+    <td>https://realbooru.com/</td>
+    <td>Favorites, Pools, Posts, Tag Searches</td>
+    <td></td>
+</tr>
 <tr>
     <td>Reddit</td>
     <td>https://www.reddit.com/</td>
@@ -1213,12 +1219,6 @@ Consider all listed sites to potentially be NSFW.
 <tr>
     <td colspan="4"><strong>Gelbooru Beta 0.2</strong></td>
 </tr>
-<tr>
-    <td>Realbooru</td>
-    <td>https://realbooru.com/</td>
-    <td>Favorites, Pools, Posts, Tag Searches</td>
-    <td></td>
-</tr>
 <tr>
     <td>Rule 34</td>
     <td>https://rule34.xxx/</td>