Portal only renders children after mounted

[giladgray]

Fixes #1901

componentDidMount() {
    // The portal element is inserted in the DOM tree after
    // the Modal's children are mounted, meaning that children
    // will be mounted on a detached DOM node. If a child
    // component requires to be attached to the DOM tree
    // immediately when mounted, for example to measure a
    // DOM node, or uses 'autoFocus' in a descendant, add
    // state to Modal and only render the children when Modal
    // is inserted in the DOM tree.
    modalRoot.appendChild(this.el);
}

(https://reactjs.org/docs/portals.html#event-bubbling-through-portals)

[giladgray]

not needed as overlayWillOpen is invoked shortly afterward for first render and then for every subsequent open, whereas this is only called after initial mount.

[giladgray]

this should happen when not inline too, so I moved it to a more logical order in the method.

[blueprint-bot]

Select autoFocus on InputGroup

Preview: documentation