Added email fallback for selected member details on activity screen

no issue

- if a member has no name when they are selected on the member activity screen we weren't showing anything in the header breadcrumbs or member filter
- switched to showing name or email

ghost/admin/app/components/members-activity/member-filter.hbs

@@ -1,7 +1,7 @@
 {{#if @selected}}
     <div class="gh-btn" {{on "click" (fn @onChange null)}}>
         <span class="gh-btn-label-green">
-            {{@selected.name}}
+            {{or @selected.name @selected.email}}
             &times;
         </span>
     </div>

ghost/admin/app/templates/members-activity.hbs

@@ -4,7 +4,7 @@
             <LinkTo @route="members-activity" @query={{reset-query-params "members-activity"}} data-test-link="members-activity-back">Activity</LinkTo>
             {{#if this.memberRecord}}
                 <span>{{svg-jar "arrow-right"}}</span>
-                <span class="truncate">{{this.memberRecord.name}}</span>
+                <span class="truncate">{{or this.memberRecord.name this.memberRecord.email}}</span>
             {{/if}}
         </h2>