[RN] add support for inviting participants during a call on mobile

* Button conditionally shown based on if the feature is enabled and available
* Hooks for launching the invite UI (delegates to the native layer)
* Hooks for using the search and dial out checks from the native layer (calls back into JS)
* Hooks for handling sending invites and passing any failures back to the native layer
* Android and iOS handling for those hooks

Author: Ryan Peck <rpeck@atlassian.com>
Author: Eric Brynsvold <ebrynsvold@atlassian.com>

react/features/invite/components/InviteButton.native.js

@@ -0,0 +1,91 @@
+// @flow
+
+import React, { Component } from 'react';
+import { connect } from 'react-redux';
+
+import { launchNativeInvite } from '../../mobile/invite-search';
+import { ToolbarButton } from '../../toolbox';
+
+/**
+ * The type of {@link EnterPictureInPictureToobarButton}'s React
+ * {@code Component} props.
+ */
+type Props = {
+
+    /**
+     * Indicates if the "Add to call" feature is available.
+     */
+    enableAddPeople: boolean,
+
+    /**
+     * Indicates if the "Dial out" feature is available.
+     */
+    enableDialOut: boolean,
+
+    /**
+     * Launches native invite dialog.
+     *
+     * @protected
+     */
+    onLaunchNativeInvite: Function,
+};
+
+/**
+ * Implements a {@link ToolbarButton} to enter Picture-in-Picture.
+ */
+class InviteButton extends Component<Props> {
+
+    /**
+     * Implements React's {@link Component#render()}.
+     *
+     * @inheritdoc
+     * @returns {ReactElement}
+     */
+    render() {
+        const {
+            enableAddPeople,
+            enableDialOut,
+            onLaunchNativeInvite,
+            ...props
+        } = this.props;
+
+        if (!enableAddPeople && !enableDialOut) {
+            return null;
+        }
+
+        return (
+            <ToolbarButton
+                iconName = { 'add' }
+                onClick = { onLaunchNativeInvite }
+                { ...props } />
+        );
+    }
+}
+
+/**
+ * Maps redux actions to {@link InviteButton}'s React
+ * {@code Component} props.
+ *
+ * @param {Function} dispatch - The redux action {@code dispatch} function.
+ * @returns {{
+*      onLaunchNativeInvite
+ * }}
+ * @private
+ */
+function _mapDispatchToProps(dispatch) {
+    return {
+
+        /**
+         * Launches native invite dialog.
+         *
+         * @private
+         * @returns {void}
+         * @type {Function}
+         */
+        onLaunchNativeInvite() {
+            dispatch(launchNativeInvite());
+        }
+    };
+}
+
+export default connect(undefined, _mapDispatchToProps)(InviteButton);