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/backend-config/reducers.ts

@@ -17,6 +17,10 @@ export const initialState: BackendConfig = {
     email: false,
     openid: false
   },
+  banner: {
+    text: '',
+    timestamp: ''
+  },
   customAuthNames: {
     ldap: '',
     oauth2: '',