Add referrer (request source) to global context (#486)

https://eaflood.atlassian.net/browse/WATER-4155

In our work building the new bill page, we are required to add a 'back' link. One of the things we have to deal with though is that you can come to the page from 3 different sources

- bill run summary
- billing account summary
- billing account all bills

The previous team solved this by using a bit of JavaScript. Simply set `javascript:history.back()` as the `href` and job done. We can't blame them for this one as it's a solution a number of digital services use.

But it's a bit of a cheat. For a start, we're supposed to support users accessing a service with JavaScript disabled. That immediately breaks all your backlinks. Plus, if you've ever found your journey through a site go a bit screwy, for example, you find yourself going in a loop, chances are it's because of backlinks implemented this way.

Ideally, the request you generate would tell the handler where to go back to if there is more than one possibility. But as we're dealing with requests from the legacy app that's not really feasible.

Fortunately, either the browser or Hapi is coming to the rescue (I'm not sure who so both get credit). The headers in the request normally include [Referer](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referer) (yes that's a misspelling - see [Wikipedia](https://en.wikipedia.org/wiki/HTTP_referer#Etymology)).

Hapi provides this in [request.info.referrer](https://hapi.dev/api/?v=21.3.2#-requestinfo). If we have that, we have our 'back' link address.

This change tweaks the global context in our `ViewsPlugin` to include this value so it is available to all pages. We also add a new `back-link.njk` which shows how we can use it to generate our backlinks.

> Note - yes, we also fall back to the JavaScript hack if the value is unavailable. No one is perfect!

app/plugins/views.plugin.js

@@ -91,6 +91,8 @@ function context (request) {
   return {
     // `assetPath` is referred to in layout.njk as the path to get static assets like client-side javascript
     assetPath: '/assets',
+    // this is the url of where the request came from. We use it to generate back links in our pages
+    referrer: request.info.referrer,
     auth: {
       authenticated: request.auth.isAuthenticated,
       authorized: request.auth.isAuthorized,

app/views/includes/back-link.njk

@@ -0,0 +1,11 @@
+{% if referrer %}
+  {% set backlink = referrer %}
+{% else %}
+  {% set backlink = 'javascript:history.back()' %}
+{% endif %}
+{{
+  govukBackLink({
+    text: "Back",
+    href: backlink
+  })
+}}