Added loading state for member list rows

no issue

- with the sparsely loaded list you can scroll to an area which is being loaded in the background
- if a list row is being loaded, display a "loading" version of the row so it doesn't look like you're stuck on a blank screen

ghost/admin/app/components/gh-members-list-item.hbs

@@ -1,4 +1,10 @@
-<li class="gh-list-row gh-members-list-item" ...attributes>
+<li class="gh-list-row gh-members-list-item {{if @member.is_loading "loading"}}" ...attributes>
+    {{#if @member.is_loading}}
+        <div class="gh-list-data gh-list-loadingcell gh-list-cellwidth-100">
+            <div class="gh-list-loading-title"></div>
+            <div class="gh-list-loading-detail"></div>
+        </div>
+    {{else}}
         <LinkTo @route="member" @model={{@member}} title="Member details" class="gh-list-data gh-members-list-basic">
             <div class="flex items-center">
                 <GhMemberAvatar @member={{@member}} @containerClass="w9 h9 mr3" />
@@ -34,4 +40,5 @@
                 <span class="nr2">{{svg-jar "arrow-right" class="w6 h6 fill-midgrey pa1"}}</span>
             </div>
         </LinkTo>
+    {{/if}}
 </li>

ghost/admin/app/templates/members.hbs

@@ -66,7 +66,7 @@
                             <div class="gh-list-header gh-list-cellwidth-20 nowrap">Created</div>
                             <div class="gh-list-header gh-list-cellwidth-chevron"></div>
                         </li>
-                        <VerticalCollection @items={{this.members}} @key="id" @containerSelector=".gh-main" @estimateHeight={{69}} @staticHeight={{true}} @bufferSize={{5}} as |member|>
+                        <VerticalCollection @items={{this.members}} @key="id" @containerSelector=".gh-main" @estimateHeight={{69}} @staticHeight={{true}} @bufferSize={{20}} as |member|>
                             <GhMembersListItem @member={{member}} @data-test-member-id={{member.id}} />
                         </VerticalCollection>
                     {{else}}