message: first impl of quote formatting
diff: @@ -478,7 +478,30 @@ export default class BasicMessageEditor extends React.Component {
  }
  _formatQuote = () => {
  -
  + const {model} = this.props;
  + const {partCreator} = this.props.model;
  + this._replaceSelection(range => {
  + const parts = range.parts;
  + parts.splice(0, 0, partCreator.plain("> "));
  + const startsWithPartial = range.start.offset !== 0;
  + const isFirstPart = range.start.index === 0;
  + const previousIsNewline = !isFirstPart && model.parts[range.start.index - 1].type === "newline";
  + // prepend a newline if there is more text before the range on this line
  + if (startsWithPartial || (!isFirstPart && !previousIsNewline)) {
  + parts.splice(0, 0, partCreator.newline());
  + }
  + // start at position 1 to make sure we skip the potentially inserted newline above,
  + // as we already inserted a quote sign for it above
  + for (let i = 1; i < parts.length; ++i) {
  + const part = parts[i];
  + if (part.type === "newline") {
  + parts.splice(i + 1, 0, partCreator.plain("> "));
  + }
  + }
  + parts.push(partCreator.newline());
  + parts.push(partCreator.newline());
  + return parts;
  + });
  }
  _formatCodeBlock = () => {
  