http: optimize HTTP request thread we don't need a mutex at all, in fact all we need is an atomic boolean to signify whether the thread is cancelled. curl options are now for the most part handled by a separate function to keep them in sync between non-threaded and threaded implementations

// Alerts

.alert-variant(@background; @border; @text-color) {
  background-color: @background;
  border-color: @border;
  color: @text-color;

  hr {
    border-top-color: darken(@border, 5%);
  }
  .alert-link {
    color: darken(@text-color, 10%);
  }
}