Added alpha feature flag for source attribution

- sets up feature flag for source attribution features

ghost/admin/app/services/feature.js

@@ -66,6 +66,7 @@ export default class FeatureService extends Service {
     @feature('searchHelper') searchHelper;
     @feature('emailAlerts') emailAlerts;
     @feature('emailClicks') emailClicks;
+    @feature('sourceAttribution') sourceAttribution;
 
     _user = null;
 

ghost/admin/app/templates/settings/labs.hbs

@@ -239,6 +239,19 @@
                         </div>
                     </div>
                 </div>
+                <div class="gh-expandable-block">
+                    <div class="gh-expandable-header">
+                        <div>
+                            <h4 class="gh-expandable-title">Source Attribution</h4>
+                            <p class="gh-expandable-description">
+                               Track which external sources are driving Member Subscriptions & Paid Conversions (Needs Member Attribution flag)
+                            </p>
+                        </div>
+                        <div class="for-switch">
+                           <GhFeatureFlag @flag="sourceAttribution" />
+                        </div>
+                    </div>
+                </div>
                 <div class="gh-expandable-block">
                     <div class="gh-expandable-header">
                         <div>

ghost/core/core/shared/labs.js

@@ -33,7 +33,8 @@ const BETA_FEATURES = [
 const ALPHA_FEATURES = [
     'urlCache',
     'beforeAfterCard',
-    'emailClicks'
+    'emailClicks',
+    'sourceAttribution'
 ];
 
 module.exports.GA_KEYS = [...GA_FEATURES];