[danbooru] inherit from BaseExtractor

- merge danbooru and e621 code
- support booru.allthefallen.moe (closes #2283)
- remove support for old e621 tag search URLs

docs/supportedsites.md

@@ -115,12 +115,6 @@ Consider all sites to be NSFW unless otherwise known.
     <td>Albums</td>
     <td></td>
 </tr>
-<tr>
-    <td>Danbooru</td>
-    <td>https://danbooru.donmai.us/</td>
-    <td>Pools, Popular Images, Posts, Tag Searches</td>
-    <td>Supported</td>
-</tr>
 <tr>
     <td>Desktopography</td>
     <td>https://desktopography.net/</td>
@@ -145,12 +139,6 @@ Consider all sites to be NSFW unless otherwise known.
     <td>Favorites, Galleries, Search Results</td>
     <td>Supported</td>
 </tr>
-<tr>
-    <td>e621</td>
-    <td>https://e621.net/</td>
-    <td>Favorites, Pools, Popular Images, Posts, Tag Searches</td>
-    <td>Supported</td>
-</tr>
 <tr>
     <td>Eka's Portal</td>
     <td>https://aryion.com/</td>
@@ -860,6 +848,28 @@ Consider all sites to be NSFW unless otherwise known.
     <td></td>
 </tr>
 
+<tr>
+    <td colspan="4"><strong>Danbooru Instances</strong></td>
+</tr>
+<tr>
+    <td>Danbooru</td>
+    <td>https://danbooru.donmai.us/</td>
+    <td>Pools, Popular Images, Posts, Tag Searches</td>
+    <td>Supported</td>
+</tr>
+<tr>
+    <td>e621</td>
+    <td>https://e621.net/</td>
+    <td>Favorites, Pools, Popular Images, Posts, Tag Searches</td>
+    <td>Supported</td>
+</tr>
+<tr>
+    <td>ATFBooru</td>
+    <td>https://booru.allthefallen.moe/</td>
+    <td>Pools, Popular Images, Posts, Tag Searches</td>
+    <td>Supported</td>
+</tr>
+
 <tr>
     <td colspan="4"><strong>Gelbooru Beta 0.1.11</strong></td>
 </tr>