A minor bug was introduced in #32184 where we didn't check if the
channel name would cause a broken link syntax (and hence should
generate a fallback md link). This would cause problems when
implementing a shortcut syntax to link to current channel.
Fixes part of #31420.
These characters cause the fallback markdown
links produced (#30071) to be broken.
Broken links are not produced when these are present
in `#**channel>topic**` syntax. It is only a problem
with the fallback markdown links.
In the function `get_stream_topic_link_syntax`,
we used the typed syntax text to make out
the channel name and then use it to
generate the appropriate link syntax.
We change that to directly accepting the
channel name.
This is in preparation for #31420
