help_docs: Extend options for getting links to Zulip content.

Extends the linking to Zulip documentation to cover:

- Getting URLs to messages via the message timestamp.
- Getting links to topics via the three-dots menu.
- Getting links to streams via right-click context menu.

Creates a new tabbed section for using the browser
address bar to copy URLs.

zerver/lib/markdown/tabbed_sections.py

@@ -78,6 +78,7 @@ TAB_SECTION_LABELS = {
     "private-streams": "Private streams",
     "via-user-profile": "Via the user's profile",
     "via-organization-settings": "Via organization settings",
+    "via-browser-address-bar": "Via browser's address bar",
 }