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>