Allow line breaks, more markdown, etc. in channel topics.

And ideally some extra markdown would be really useful.

• Line breaks /n 
• headings: h1., h2. or #, ##, or something.
• bullets / lists 
• linktext  {https://thislink.tld|this-text} sort of thing.

This would help when people want to use the channel summary to share common resources for topics, external assets and doc libraries, rules/guidelines of the channel, etc. 

Thanks!