Fixed 403 Forbidden thrown when getting Website metadata

- Add User-Agent to header to prevent 403 Forbidden when downloading webpage strings
- Add Bookmark URL and User Id unique combo constraint
- Add Tag Name and User Id unique combo constraint

YABA.Data/Context/YABABaseContext.cs

@@ -39,7 +39,11 @@ namespace YABA.Data.Context
                 .IsUnique();
 
             modelBuilder.Entity<Tag>()
-                .HasIndex(x => x.Name)
+                .HasIndex(x => new { x.Name, x.UserId })
+                .IsUnique();
+
+            modelBuilder.Entity<Bookmark>()
+                .HasIndex(x => new { x.Url, x.UserId })
                 .IsUnique();
         }