policies.json: configure firefox home (MR 19)

Disable everything but the search bar. This gives a clean page, of a
cog wheel on the top right where the user can adjust it as they want
(e.g. enable most frequently visited pages), then the FF logo and the
search bar.

I've considered enabling TopSites in order to show the most frequently
visited sites according to the browser's history. However, this also
enables built-in links to youtube, facebook, reddit, amazon, twitter.
And while it is convenient to have links you frequently visit available
(based on your history, not these built-ins!), getting an almost blank
page instead is actually a good idea to reduce the bad pattern of
opening a web browser and then forgetting your original intention until
it opens, and instead surfing mindlessly on the most recently visited
websites. The almost blank page gives the user a moment to think about
what they really want to do.

src/policies.json

@@ -15,6 +15,15 @@
             "StartPage": "homepage"
         },
 
+        "FirefoxHome": {
+            "Search": true,
+            "TopSites": false,
+            "Highlights": false,
+            "Pocket": false,
+            "Snippets": false,
+            "Locked": false
+        },
+
         "SearchEngines": {
             "Default": "DuckDuckGo",
             "Remove": [