Recognize "channel" as operator in narrow links

[chrisbobbe]

In zulip/zulip#28468, we plan to rename "stream" to "channel" in the Zulip UI.

We should start to recognize narrow links with a "channel" operator, while continuing to recognize the "stream" operator (because links with the old operator will be present in existing messages). So, for example, the app should parse both of these URLs:

https://chat.zulip.org/#narrow/stream/378-api-design/topic/URLs.20for.20stream.20to.20channel.20rename
https://chat.zulip.org/#narrow/channel/378-api-design/topic/URLs.20for.20stream.20to.20channel.20rename

Discussion: https://chat.zulip.org/#narrow/stream/378-api-design/topic/URLs.20for.20stream.20to.20channel.20rename/near/1784294

Related issues:

• Use "stream" or "channel" based on server version #5827 (for UI strings)
• Generate narrow links using "channel" operator, not "stream", if server is recent #5861 (writing as opposed to reading narrow links)