[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

test/results/discord.py

@@ -0,0 +1,64 @@
+# -*- coding: utf-8 -*-
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+
+from gallery_dl.extractor import discord
+
+
+__tests__ = (
+{
+    "#url"     : "https://discord.com/channels/302094807046684672/1306705919916249098",
+    "#category": ("", "discord", "channel"),
+    "#class"   : discord.DiscordChannelExtractor,
+#    # access token & access to minecraft server required for this test (REMEMBER TO REMOVE TOKEN BEFORE COMMITTING)
+#    "#range"   : "1-2",
+#    "#count"   : 2,
+#    "#options" : {"token": ""},
+#
+#    "#server"       : "MINECRAFT",
+#    "#server_id"    : "302094807046684672",
+#    "#server_files" : list,
+#    "#owner_id"     : "827254075857829920",
+#    "#channel"      : str,
+#    "#channel_id"   : str,
+#    "#channel_type" : 11,
+#    "#channel_topic": str,
+#    "#parent"       : "challenges",
+#    "#parent_id"    : "1306705919916249098",
+#    "#parent_type"  : 15,
+#    "#is_thread"    : True,
+#
+#    "author"      : str,
+#    "author_id"   : str,
+#    "author_files": list,
+#    "message"     : str,
+#    "message_id"  : str,
+#    "type"        : str,
+#    "date"        : "type:datetime",
+#    "files"       : list,
+#    "filename"    : str,
+#    "extension"   : str,
+#    "num"         : int,
+},
+
+{
+    "#url"     : "https://discord.com/channels/302094807046684672/1306705919916249098/threads/1306706528786583623",
+    "#category": ("", "discord", "channel"),
+    "#class"   : discord.DiscordChannelExtractor,
+},
+
+{
+    "#url"     : "https://discord.com/channels/302094807046684672",
+    "#category": ("", "discord", "server"),
+    "#class"   : discord.DiscordServerExtractor,
+},
+
+{
+    "#url"     : "https://discord.com/channels/@me/302094807046684672",
+    "#category": ("", "discord", "direct-messages"),
+    "#class"   : discord.DiscordDirectMessagesExtractor,
+},
+
+)