Copy emoji in the subject from parent post

Sometimes people put emoji in the subject, which results in the subject
looking broken if someone replies to it from a server that does not
have the said emoji under the same shortcode. This patch solves the problem
by extending the emoji set available in the summary to that of the parent
post.

test/fixtures/tesla_mock/emoji-in-summary.json

@@ -0,0 +1,49 @@
+{
+  "@context": [
+    "https://www.w3.org/ns/activitystreams",
+    "https://patch.cx/schemas/litepub-0.1.jsonld",
+    {
+      "@language": "und"
+    }
+  ],
+  "actor": "https://patch.cx/users/rin",
+  "attachment": [],
+  "attributedTo": "https://patch.cx/users/rin",
+  "cc": [
+    "https://patch.cx/users/rin/followers"
+  ],
+  "content": ":joker_disapprove: <br><br>just grabbing a test fixture, nevermind me",
+  "context": "https://patch.cx/contexts/2c3ce4b4-18b1-4b1a-8965-3932027b5326",
+  "conversation": "https://patch.cx/contexts/2c3ce4b4-18b1-4b1a-8965-3932027b5326",
+  "id": "https://patch.cx/objects/a399c28e-c821-4820-bc3e-4afeb044c16f",
+  "published": "2021-03-22T16:54:46.461939Z",
+  "sensitive": null,
+  "source": ":joker_disapprove: \r\n\r\njust grabbing a test fixture, nevermind me",
+  "summary": ":joker_smile: ",
+  "tag": [
+    {
+      "icon": {
+        "type": "Image",
+        "url": "https://patch.cx/emoji/custom/joker_disapprove.png"
+      },
+      "id": "https://patch.cx/emoji/custom/joker_disapprove.png",
+      "name": ":joker_disapprove:",
+      "type": "Emoji",
+      "updated": "1970-01-01T00:00:00Z"
+    },
+    {
+      "icon": {
+        "type": "Image",
+        "url": "https://patch.cx/emoji/custom/joker_smile.png"
+      },
+      "id": "https://patch.cx/emoji/custom/joker_smile.png",
+      "name": ":joker_smile:",
+      "type": "Emoji",
+      "updated": "1970-01-01T00:00:00Z"
+    }
+  ],
+  "to": [
+    "https://www.w3.org/ns/activitystreams#Public"
+  ],
+  "type": "Note"
+}