Add post and event cards, shared calendar sheet and per-event ICS; rework past events and next-event card

Elternbeirat.Web/Features/Posts/PostCard.razor

@@ -0,0 +1,13 @@
+@*
+    One post as a card that links to the post: date, title, a teaser from the body
+    and "Weiterlesen". The arrow is a hint where the card leads; it does not move.
+*@
+<a class="@CssClass" href="@Href">
+    <span class="post-date"><Icon Name="calendar" Size="16" /><PostDate Value="Value.Date" /></span>
+    <span class="post-title">@Value.Title</span>
+    @if (Teaser.Length > 0)
+    {
+        <span class="post-teaser">@Teaser</span>
+    }
+    <span class="post-more">Weiterlesen <Icon Name="arrow-right" Size="16" /></span>
+</a>