[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

scripts/supportedsites.py

@@ -235,6 +235,9 @@ SUBCATEGORY_MAP = {
         "status": "Status Updates",
         "watch-posts": "",
     },
+    "discord": {
+        "direct-messages": "DMs",
+    },
     "fanbox": {
         "supporting": "Supported User Feed",
         "redirect"  : "Pixiv Redirects",