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/Migrations/YABABaseContextModelSnapshot.cs

@@ -67,6 +67,10 @@ namespace YABA.Data.Migrations
                     b.HasIndex("UserId")
                         .HasDatabaseName("ix_bookmarks_user_id");
 
+                    b.HasIndex("Url", "UserId")
+                        .IsUnique()
+                        .HasDatabaseName("ix_bookmarks_url_user_id");
+
                     b.ToTable("bookmarks");
                 });
 
@@ -113,13 +117,13 @@ namespace YABA.Data.Migrations
                     b.HasKey("Id")
                         .HasName("pk_tags");
 
-                    b.HasIndex("Name")
-                        .IsUnique()
-                        .HasDatabaseName("ix_tags_name");
-
                     b.HasIndex("UserId")
                         .HasDatabaseName("ix_tags_user_id");
 
+                    b.HasIndex("Name", "UserId")
+                        .IsUnique()
+                        .HasDatabaseName("ix_tags_name_user_id");
+
                     b.ToTable("tags");
                 });