Added publication icon to members site data

refs https://github.com/TryGhost/members.js/issues/72

- Portal is using using publication logo from settings for signup/signin pages
- Instead, we are switching to using publication icon from settings, which also needs to be passed in site data API

core/server/services/members/middleware.js

@@ -90,6 +90,7 @@ const getMemberSiteData = async function (req, res) {
         title: settingsCache.get('title'),
         description: settingsCache.get('description'),
         logo: settingsCache.get('logo'),
+        icon: settingsCache.get('icon'),
         accent_color: settingsCache.get('accent_color'),
         url: urlUtils.urlFor('home', true),
         version: ghostVersion.safe,