Fixed conflicting svg ids

no issue

- enables `prefixIds` svgo option to guarantee all `id` attributes are unique across svgs
- disabled `cleanupIds` option so that it doesn't interfere with id uniqueness
- fixes instagram icon background not showing

ghost/admin/ember-cli-build.js

@@ -200,6 +200,8 @@ module.exports = function (defaults) {
             ],
             optimizer: {
                 plugins: [
+                    {prefixIds: true},
+                    {cleanupIds: false},
                     {removeDimensions: true},
                     {removeTitle: true},
                     {removeXMLNS: true},