Updated admin page to be more streamlined and added the beginning of the blogging features

OpenArchival.Blazor.Blog/BlogPostNotFound.razor

@@ -0,0 +1,17 @@
+@namespace OpenArchival.Blazor.Blog
+
+@using MudBlazor
+@page "/blog-not-found"
+@page "/blog-not-found/{Message}"
+
+<MudPaper Class="pa-4 ma-2 rounded" Elevation="3">
+    <MudText Typo="Typo.h1" Align="Align.Center">404: Post Not Found!</MudText>
+    @if (!string.IsNullOrEmpty(Message)) {
+        <MudText Typo="Typo.body1" Align="Align.Center">@Message</MudText>
+    }
+</MudPaper>
+
+@code {
+    [Parameter]
+    public string? Message { get; set; }
+}