Shelter donations result in wrong internal balance if fee-on-transfer token is used

[CloudEllie]

Judge @GalloDaSballo has assessed the 2nd Low-severity item in QA Report #66 as Medium risk. The relevant finding follows:

…

In case of a fee-on-transfer ERC20 token the savedTokens balance will not represent the actual balance of the contract:

• https://github.com/code-423n4/2022-02-concur/blob/main/contracts/Shelter.sol#L34-L35

You can either disable the use of fee-on-transfer tokens for donations or use the actual balance:

// disable
uint oldBalance = _token.balanceOf(address(this));
_token.safeTransferFrom(msg.sender, address(this), _amount);
uint newBalance = _token.balanceOf(address(this));
require(_amount == (newBalance - oldBalance));
savedTokens[_token] += _amount;

// use actual balance
_token.safeTransferFrom(msg.sender, address(this), _amount);
savedTokens[_token] = _token.balanceOf(address(this));

[CloudEllie]

Duplicate of #180