[🪲BUG] Number of digits in RH crypto order is not checked

[tfukaza]

Describe the bug
Sometimes, when placing crypto orders on Robinhood, an error is returned by the API indicating the the number of digits in the order quantity is too much.

To Reproduce
For example, if you try to buy 0.00000001 DOGE, the order will fail.

Expected behavior
It appears that on Robinhood, the number of digits D_o allowed in the order quantity is N - D_p, where N is some constant and D_p is the number of digits in the current price of the cryptocurrency.

To fix this bug, you must:

• Investigate the value of N
• Add checks to buy/sell functions so order quantities are automatically rounded down to the proper number of digits.