Updates `.md` files in api_docs/ to use "stream" instead of "channel"
for descriptive text, with the exception of the API changelog file.
Part of stream to channel rename project.
Adds "channel" to the `stream_wildcards` frozenset for stream
wildcard notifications on the backend/server.
Updates frontend/web-app to handle "channel" as the other stream
wildcards are handled in the typeahead and composebox modules.
Updates the API version and documentation for the addition of
"channel" as a wildcard mention. But does not change any of the
functionailty of (or deprecate) the "stream" wildcard at this
point.
Part of project to rename "stream" to "channel".
Adds a "Message formatting" article to the API documentation. To
start, the article is a changelog of updates to markup features for
messages, with the idea being that it can be expanded to further
describe and document Zulip's markup features in future iterations.
