integrations: Add URL option and UI for mapping messages to Zulip channels.

This commit adds a "mapping" URL option preset that adds "Matching Zulip
channel" option to the stream dropdown widget. When that option is
chosen from the dropdown, it adds another parameter to the integration
URL -- "&mapping=channels".

This "mapping" parameter is meant to be used by integrations like Slack
to identify whether the user wants to map Slack channels to different
Zulip channels or different topics within a single channel.

This adds an icon for the `mapping`s' drop down option in the "Where to
send notification" drop down field.

Co-authored-by: Pieter CK <pieterceka123@gmail.com>
Co-authored-by: Lauryn Menard <lauryn@zulip.com>

api_docs/incoming-webhooks-walkthrough.md

@@ -304,6 +304,15 @@ Currently configured preset URL options:
   `ignore_private_repositories` boolean parameter will be added to the
   [generated integration URL](/help/generate-integration-url).
 
+- **`MAPPING`**: This preset is intended to be used for [chat-app
+  integrations](/integrations/communication) (like Slack), and adds a
+  special option, **Matching Zulip channel**, to the UI for where to send
+  Zulip notification messages. This special option maps the notification
+  messages to Zulip channels that match the messages' original channel
+  name in the third-party app. When selected, this requires setting a
+  single topic for notification messages, and adds `&mapping=channels`
+  to the [generated integration URL](/help/generate-integration-url).
+
 ## Step 4: Manually testing the webhook
 
 For either one of the command line tools, first, you'll need to get an