Add link to Open Manual (Configurable)

Each project should configure this to point to their page on the open manual. (i.e. https://github.com/colouring-cities/manual/wiki/M3.-COLOURING-BRITAIN)

app/src/cc-config.json

@@ -1,7 +1,9 @@
 {
     "cityName": "Cities",
     "projectBlurb": "Colouring {City Name} is part Colouring Cities.",
+    
     "githubURL": "https://github.com/colouring-cities/colouring-core",
+    "manualURL": "https://github.com/colouring-cities/manual/wiki/M3.-COLOURING-BRITAIN",
     "privacyStatement": "{Privacy statement goes here}",
 
     "initialMapPosition": [ 51.5245255, -0.1338422 ],

app/src/cc-config.ts

@@ -1,8 +1,12 @@
+import { StringNullableChain } from "lodash";
+
 export interface CCConfig
 {
     cityName: string;                           // City name (i.e. "Colouring {City Name}")
     projectBlurb: string;                       // Description used on homepage
+    
     githubURL: string;                          // URL of the project's GitHub repository
+    manualURL: string;                          // Link to the project's page in the Open Manual (i.e. https://github.com/colouring-cities/manual/wiki/M3.-COLOURING-BRITAIN)
     privacyStatement: string;                   // Privacy statement, including where data is stored
     
     initialMapPosition: [number, number];       // Initial location of the map [latitude, longitude]

app/src/frontend/header.tsx

@@ -78,6 +78,11 @@ function getCurrentMenuLinks(username: string): MenuLink[][] {
                 text: "About the Colouring Cities Research Programme",
                 external: true
             },
+            {
+                to: config.manualURL,
+                text: "About the Colouring " + config.cityName + " Project",
+                external: true
+            },
             {
                 to: "https://github.com/colouring-cities/manual/wiki/A2.-How-to%3F-Guides",
                 text: "How to Use",