integrations: Improve GitHub issue labeled and unlabeled notifications.

Earlier, the notifications had no information about the labels
being added or removed.

zerver/lib/webhooks/git.py

@@ -50,6 +50,9 @@ CREATE_BRANCH_MESSAGE_TEMPLATE = "{user_name} created [{branch_name}]({url}) bra
 CREATE_BRANCH_WITHOUT_URL_MESSAGE_TEMPLATE = "{user_name} created {branch_name} branch."
 REMOVE_BRANCH_MESSAGE_TEMPLATE = "{user_name} deleted branch {branch_name}."
 
+ISSUE_LABELED_OR_UNLABELED_MESSAGE_TEMPLATE = "[{user_name}]({user_url}) {action} the [{label_name}]({label_url}) label {preposition} [Issue #{id}]({url})."
+ISSUE_LABELED_OR_UNLABELED_MESSAGE_TEMPLATE_WITH_TITLE = "[{user_name}]({user_url}) {action} the [{label_name}]({label_url}) label {preposition} [Issue #{id} {title}]({url})."
+
 PULL_REQUEST_OR_ISSUE_MESSAGE_TEMPLATE = "{user_name} {action} [{type}{id}]({url})"
 PULL_REQUEST_OR_ISSUE_MESSAGE_TEMPLATE_WITH_TITLE = (
     "{user_name} {action} [{type}{id} {title}]({url})"
@@ -273,6 +276,32 @@ def get_issue_event_message(
     )
 
 
+def get_issue_labeled_or_unlabeled_event_message(
+    user_name: str,
+    action: str,
+    url: str,
+    number: int,
+    label_name: str,
+    label_url: str,
+    user_url: str,
+    title: Optional[str] = None,
+) -> str:
+    args = {
+        "user_name": user_name,
+        "action": action,
+        "url": url,
+        "id": number,
+        "label_name": label_name,
+        "label_url": label_url,
+        "user_url": user_url,
+        "title": title,
+        "preposition": "to" if action == "added" else "from",
+    }
+    if title is not None:
+        return ISSUE_LABELED_OR_UNLABELED_MESSAGE_TEMPLATE_WITH_TITLE.format(**args)
+    return ISSUE_LABELED_OR_UNLABELED_MESSAGE_TEMPLATE.format(**args)
+
+
 def get_push_tag_event_message(
     user_name: str, tag_name: str, tag_url: Optional[str] = None, action: str = "pushed"
 ) -> str: