[boosty] added new direct message extractor

- formatting
- fixed linting formatting errors
- fixed E999 SyntaxError: invalid syntax
- fixed class naming
- fixed mandatory extractor.boosty.metadata as true requirement
- update
  - apply changes
  - add test
  - update docs/supportedsites
- improve 'dialog' pagination logic

docs/supportedsites.md

@@ -148,7 +148,7 @@ Consider all listed sites to potentially be NSFW.
 <tr>
     <td>Boosty</td>
     <td>https://www.boosty.to/</td>
-    <td>Subscriptions Feed, Followed Users, Media Files, Posts, User Profiles</td>
+    <td>DMs, Subscriptions Feed, Followed Users, Media Files, Posts, User Profiles</td>
     <td><a href="https://github.com/mikf/gallery-dl#cookies">Cookies</a></td>
 </tr>
 <tr>