Spreadsheet: Save the cells under the correct name

This broke in 6a6f19a72fe15c05c39a53fa5be2b4ca3d65edf3, which replaced
the representation of columns with numbers.
As a result, the save logic would store cells as
"\x<column_index><row_number>", which is obviously wrong.
Fixes #5905.
Also simplifies the control flow in `import_worksheet` a bit.

Userland/Applications/Spreadsheet/ImportDialog.cpp

@@ -244,11 +244,8 @@ Result<NonnullRefPtrVector<Sheet>, String> ImportDialog::make_and_run_for(String
             if (!sheet_json.is_object())
                 return IterationDecision::Continue;
 
-            auto sheet = Sheet::from_json(sheet_json.as_object(), workbook);
-            if (!sheet)
-                return IterationDecision::Continue;
-
-            sheets.append(sheet.release_nonnull());
+            if (auto sheet = Sheet::from_json(sheet_json.as_object(), workbook))
+                sheets.append(sheet.release_nonnull());
 
             return IterationDecision::Continue;
         });

Userland/Applications/Spreadsheet/Spreadsheet.cpp

@@ -544,7 +544,7 @@ JsonObject Sheet::to_json() const
     JsonObject cells;
     for (auto& it : m_cells) {
         StringBuilder builder;
-        builder.append(it.key.column);
+        builder.append(column(it.key.column));
         builder.appendff("{}", it.key.row);
         auto key = builder.to_string();