[discord] add support (#6836)

* first commit

* add --

* skip video embeds

* fix typo

* removed ambiguity

* add category support

* code tweaks

* more reliable embed extraction

* handle 403 errors (testing done)

* added "parent_id" keyword

* added "parent", "parent_type" keywords

the extractor should be now ready to merge!

* removed unnecessary dict unpacking

* added empty text messages extraction

* added "channel_topic"

* even more metadata extraction

can now extract all embeds images & text, as well as server banners. also code is much better.

* added user avatar and banner

* better pagination

* fix regression

* minor tweaks

* Made requested changes

docs/supportedsites.md

@@ -205,6 +205,12 @@ Consider all listed sites to potentially be NSFW.
     <td>Avatars, Backgrounds, Collections, Deviations, Favorites, Folders, Followed Users, Galleries, Gallery Searches, Journals, Scraps, Search Results, Sta.sh, Status Updates, Tag Searches, User Profiles, Watches</td>
     <td><a href="https://github.com/mikf/gallery-dl#oauth">OAuth</a></td>
 </tr>
+<tr>
+    <td>Discord</td>
+    <td>https://discord.com/</td>
+    <td>Channels, DMs, Servers</td>
+    <td></td>
+</tr>
 <tr>
     <td>Dynasty Reader</td>
     <td>https://dynasty-scans.com/</td>