Allow overwriting of sharing links

This PR changes this by grouping the links within an object key: `Site.Data.sharing.links`. This means that sites can provide their own
`data/sharing.json` file and overwrite the `links` key, to provide links of their own links.

data/sharing.json

@@ -1,32 +1,34 @@
 {
-  "email": {
-    "icon": "email",
-    "title": "sharing.email",
-    "url": "mailto:?body=%s&subject=%s"
-  },
-  "facebook": {
-    "icon": "facebook",
-    "title": "sharing.facebook",
-    "url": "https://www.facebook.com/sharer/sharer.php?u=%s&quote=%s"
-  },
-  "linkedin": {
-    "icon": "linkedin",
-    "title": "sharing.linkedin",
-    "url": "https://www.linkedin.com/shareArticle?mini=true&url=%s&title=%s"
-  },
-  "pinterest": {
-    "icon": "pinterest",
-    "title": "sharing.pinterest",
-    "url": "https://pinterest.com/pin/create/bookmarklet/?url=%s&description=%s"
-  },
-  "reddit": {
-    "icon": "reddit",
-    "title": "sharing.reddit",
-    "url": "https://reddit.com/submit/?url=%s&resubmit=true&title=%s"
-  },
-  "twitter": {
-    "icon": "twitter",
-    "title": "sharing.twitter",
-    "url": "https://twitter.com/intent/tweet/?url=%s&text=%s"
+  "links": {
+    "email": {
+      "icon": "email",
+      "title": "sharing.email",
+      "url": "mailto:?body=%s&subject=%s"
+    },
+    "facebook": {
+      "icon": "facebook",
+      "title": "sharing.facebook",
+      "url": "https://www.facebook.com/sharer/sharer.php?u=%s&quote=%s"
+    },
+    "linkedin": {
+      "icon": "linkedin",
+      "title": "sharing.linkedin",
+      "url": "https://www.linkedin.com/shareArticle?mini=true&url=%s&title=%s"
+    },
+    "pinterest": {
+      "icon": "pinterest",
+      "title": "sharing.pinterest",
+      "url": "https://pinterest.com/pin/create/bookmarklet/?url=%s&description=%s"
+    },
+    "reddit": {
+      "icon": "reddit",
+      "title": "sharing.reddit",
+      "url": "https://reddit.com/submit/?url=%s&resubmit=true&title=%s"
+    },
+    "twitter": {
+      "icon": "twitter",
+      "title": "sharing.twitter",
+      "url": "https://twitter.com/intent/tweet/?url=%s&text=%s"
+    }
   }
 }