Markdown-only proposal description editor

[mhagel]

Link to Issue

Closes: #6635

Description of Changes

• Makes Cosmos Onchain Proposal description input markdown-only
• slightly widens form to make editor toolbar fit in one line

"How We Fixed It"

• Tested by copying some proposal markdown content from past Juno summaries

Test Plan

• CA (click around) tested on local:
  • http://localhost:8080/csdk-beta/new/proposal. or /csdk
  • sign in with "shared account"
  • copy/paste any markdown, or type a description in markdown
  • Check preview to make sure it looks good
  • Send Transaction -> (Keplr: set TX Fee to zero, confirm)
  • Expect resulting proposal page to look like Preview

Deployment Plan

Other Considerations

• I noticed juno proposals have \n newline characters a lot. Our editor doesn't support that, but the Preview window should resolve any confusion

[lzach83]

lgtm