Amount must ALWAYS be saved as integer.
In this case, converted_amount is also render later using MoneyHelper::format() which take the amount as an integer.

Notes:

• the migration in 2021_07_13_164157_fix_expense_converted_amount.php use a change() which does not work on sqlite (it locks the table!)
• that's why I've updated the columns in 2020_06_30_211833_create_expenses_table.php also: it will apply for sqlite, or new installation, which is fine, and the migration will not apply to existing installation, which is fine too (the 2nd migration will fix it)

Thanks for contributing.

Please make sure your PR follows those guidelines:

• Unit tests for everything.
• Documentation has been written on the documentation website.
• Dummy data in the SetupDummyAccount file, so test accounts are populated automatically with fresh data.