message: use matrix as support link
diff: namespace {
  const QString kLinkFmt = "<a href='%1'>%2</a>";
  -const QString kSupportLink = "<TODO: replace mail. mailto:support@gitahead.com";
  +const QString kSupportLink = "https://matrix.to/#/#Gittyup:matrix.org";
  const QString kVideoLink = "TODO";
  class DefaultWidget : public QFrame {
  