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/reducers/plugins/index.js

@@ -0,0 +1,22 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+import {combineReducers} from 'redux';
+import {ActionTypes} from 'utils/constants.jsx';
+
+function components(state = {}, action) {
+    switch (action.type) {
+    case ActionTypes.RECEIVED_PLUGIN_COMPONENTS: {
+        if (action.data) {
+            return {...action.data, ...state};
+        }
+        return state;
+    }
+    default:
+        return state;
+    }
+}
+
+export default combineReducers({
+    components
+});