[backend/federation] Fix GTS compatibility

Another day, another fedi software referencing an invalid/nonexistent context by link.

Iceshrimp.Backend/Core/Federation/ActivityStreams/Contexts/as-extensions.json

@@ -8,6 +8,7 @@
       "@id": "http://joinmastodon.org/ns#featured",
       "@type": "@id"
     },
-    "EmojiReact": "http://litepub.social/ns#EmojiReact"
+    "EmojiReact": "http://litepub.social/ns#EmojiReact",
+    "sensitive": "https://www.w3.org/ns/activitystreams#sensitive"
   }
 }

Iceshrimp.Backend/Core/Federation/ActivityStreams/Contexts/gts.json

@@ -0,0 +1,17 @@
+{
+  "@context": {
+    "gts": "https://gotosocial.org/ns#",
+    "interactionPolicy": "gts:interactionPolicy",
+    "canLike": "gts:canLike",
+    "canReply": "gts:canReply",
+    "canAnnounce": "gts:canAnnounce",
+    "always": {
+      "@id": "gts:always",
+      "@type": "@id"
+    },
+    "approvalRequired": {
+      "@id": "gts:approvalRequired",
+      "@type": "@id"
+    }
+  }
+}

Iceshrimp.Backend/Core/Federation/ActivityStreams/LdHelpers.cs

@@ -18,6 +18,7 @@ public static class LdHelpers
 		["https://www.w3.org/ns/activitystreams"] = GetPreloadedContext("as.json"),
 		["https://w3id.org/security/v1"]          = GetPreloadedContext("security.json"),
 		["http://joinmastodon.org/ns"]            = GetPreloadedContext("toot.json"),
+		["https://gotosocial.org/ns"]             = GetPreloadedContext("gts.json"),
 		["http://schema.org/"]                    = GetPreloadedContext("schema.json"),
 		["litepub-0.1"]                           = GetPreloadedContext("litepub.json")
 	};