feat: support float amount values, and fix debts and gifts amount forms

[asbiin]

This fix #333
This also fix #3761, storing amount in database as integer value, rather that decimal.
Every currencies don't use a 2 digits precision, and using decimal/float can lead to calculation errors.

This introduce the notions of monetary amount vs exchange value:

• exchange value is the amount displayed for the user, or entered by a user, like 12.34 (or 12,34)
• monetary amount is the amount stored in the database, so as an integer.

[djaiss]

I don't understand why this change was needed.

[asbiin]

The reason for this PR is explain in this tweet: https://twitter.com/michaeldyrynda/status/1253516294571175938

In addition, as you may now, some currency have a 2 digit precision (like $ 12.34), but others could have 3 digits (dinar), 1 digit (Cambodian Riel), or 0.
So we really should store amount as an integer (like 1234 for $12.34), and that actually the format required by the \Money\Money class. It will reduce possible bugs, complexity, and improve accuracy.

[djaiss]

😀 Go to go!

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