Placeholder when entering weight value

• When entering a weight value for an issue, there's no obvious UI indicating that you can enter any non-negative integer value.
• This issue should solve that.

Proposal

• Having a simple placeholder that tells you can enter any number, Enter a number
• Replace the HTML input type to be number, so that you can only enter numbers.
• Remove the existing red warning message since we don't need it anymore.
• Should apply to any sidebar when you are entering a weight for an issue, including the board.