message: Fix ctrl+a, backspace toggling block format
  Now it will delete the selected range (and not toggle the block format). Fixes vector-im/riot-web#4753
diff: @@ -519,7 +519,8 @@ export default class MessageComposerInput extends React.Component {
  newState = RichUtils.toggleInlineStyle(this.state.editorState, 'STRIKETHROUGH');
  } else if (shouldToggleBlockFormat) {
  const currentStartOffset = this.state.editorState.getSelection().getStartOffset();
  - if (currentStartOffset === 0) {
  + const currentEndOffset = this.state.editorState.getSelection().getEndOffset();
  + if (currentStartOffset === 0 && currentEndOffset === 0) {
  // Toggle current block type (setting it to 'unstyled')
  newState = RichUtils.toggleBlockType(this.state.editorState, currentBlockType);
  }
  