ref: move all prop type declaration to flow

For the most part the changes are taking the "static propTypes" declaration off
of components and declaring them as Flow types. Sometimes to support flow some
method signatures had to be added. There are some exceptions in which more had
to be done to tame the beast:
- AbstractVideoTrack: put in additional truthy checks for videoTrack.
- Video: add truthy checks for the _videoElement ref.
- shouldRenderVideoTrack function: Some component could pass null for the
  videoTrack argument and Flow wanted that called out explicitly.
- DisplayName: Add a truthy check for the input ref before acting on it.
- NumbersList: Move array checks inline for Flow to comprehend array methods
  could be called. Add type checks in the Object.entries loop as the value is
  assumed to be a mixed type by Flow.
- AbstractToolbarButton: add additional truthy check for passed in type.

react/features/overlay/components/ReloadButton.js

@@ -1,46 +1,37 @@
 /* @flow */
 
-import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { connect } from 'react-redux';
 
 import { reloadNow } from '../../app';
 import { translate } from '../../base/i18n';
 
+/**
+ * The type of the React {@code Component} props of {@link ReloadButton}.
+ */
+type Props = {
+
+    /**
+     * Reloads the page.
+     */
+    _reloadNow: Function,
+
+    /**
+     * The function to translate human-readable text.
+     */
+    t: Function,
+
+    /**
+     * The translation key for the text in the button.
+     */
+    textKey: string
+};
+
 /**
  * Implements a React Component for button for the overlays that will reload
  * the page.
  */
-class ReloadButton extends Component<*> {
-    /**
-     * PageReloadOverlay component's property types.
-     *
-     * @static
-     */
-    static propTypes = {
-        /**
-         * Reloads the page.
-         *
-         * @type {Function}
-         */
-        _reloadNow: PropTypes.func,
-
-        /**
-         * The function to translate human-readable text.
-         *
-         * @public
-         * @type {Function}
-         */
-        t: PropTypes.func,
-
-        /**
-         * The translation key for the text in the button.
-         *
-         * @type {string}
-         */
-        textKey: PropTypes.string.isRequired
-    };
-
+class ReloadButton extends Component<Props> {
     /**
      * Renders the button for relaod the page if necessary.
      *