Adds an info banner to the app (#190)

* added info-banner component to show the banner.text, we got from the backend config. This banner is shown on top of the landing page (intro, history, login/signup and profile) and also on top of the editor and links to `/n/banner`
* added banner to backendConfig Redux state
* added BannerState to the ApplicationState with that the showing of the banner is globally controlled, the banner text is given to the banner component and the timestamp to acknowledge a banner was read by the user
* the timestamp of a dismissed note is saved in the browsers localStorage to determine in the future if the banner should be shown

Signed-off-by: Philip Molares <philip.molares@udo.edu>
Co-authored-by: Erik Michelson <github@erik.michelson.eu>

src/redux/banner/reducers.ts

@@ -0,0 +1,22 @@
+import { Reducer } from 'redux'
+import {
+  BannerActions,
+  BannerActionType,
+  BannerState,
+  SetBannerAction
+} from './types'
+
+export const initialState: BannerState = {
+  show: true,
+  text: '',
+  timestamp: ''
+}
+
+export const BannerReducer: Reducer<BannerState, BannerActions> = (state: BannerState = initialState, action: BannerActions) => {
+  switch (action.type) {
+    case BannerActionType.SET_BANNER:
+      return (action as SetBannerAction).state
+    default:
+      return state
+  }
+}