Experimental implementation for webapp plugins (#7185)

* Start of experimental implementation for webapp plugins

* Updates to webapp plugin architecture

* Update pluggable test

* Remove debug code

webapp/plugins/pluggable/index.js

@@ -0,0 +1,17 @@
+// Copyright (c) 2017 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+import {connect} from 'react-redux';
+import {getTheme} from 'mattermost-redux/selectors/entities/preferences';
+
+import Pluggable from './pluggable.jsx';
+
+function mapStateToProps(state, ownProps) {
+    return {
+        ...ownProps,
+        components: state.plugins.components,
+        theme: getTheme(state)
+    };
+}
+
+export default connect(mapStateToProps)(Pluggable);

webapp/plugins/pluggable/pluggable.jsx

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+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+// EXPERIMENTAL - SUBJECT TO CHANGE
+
+import React from 'react';
+import PropTypes from 'prop-types';
+
+export default class Pluggable extends React.PureComponent {
+    static propTypes = {
+
+        /*
+         * Should be a single overridable React component
+         */
+        children: PropTypes.element.isRequired,
+
+        /*
+         * Components for overriding provided by plugins
+         */
+        components: PropTypes.object.isRequired,
+
+        /*
+         * Logged in user's theme
+         */
+        theme: PropTypes.object.isRequired
+    }
+
+    render() {
+        const child = React.Children.only(this.props.children).type;
+        const components = this.props.components;
+
+        if (child == null) {
+            return null;
+        }
+
+        // Include any props passed to this component or to the child component
+        let props = {...this.props};
+        Reflect.deleteProperty(props, 'children');
+        Reflect.deleteProperty(props, 'components');
+        props = {...props, ...this.props.children.props};
+
+        // Override the default component with any registered plugin's component
+        if (components.hasOwnProperty(child.name)) {
+            const PluginComponent = components[child.name];
+            return (
+                <PluginComponent
+                    {...props}
+                    theme={this.props.theme}
+                />
+            );
+        }
+
+        return React.cloneElement(this.props.children, {...props});
+    }
+}