support markdown in comments

[patte]

API change (servers/republik/graphql/schema-types.js):

type Comment {
  ...
  # maybe becomes mdast/JSON later
  content: String
  ...
}

becomes:

type Comment {
  ...
  # mdAST
  content: JSON
  # content as string
  text: String
  ...
}

submitComment stays the same.

parse supported types:

• sanitize
• github formatting
• no tables and footnotes
• breaks (https://github.com/remarkjs/remark-breaks)