web/admin: balance movements improvements

[cruzccl]

Just a few suggestions:

• Add a description field whenever a balance is modified. This description should be included along as the movement itself in the balance history subsection.

• Apart from increasing/decreasing balance, it would be nice to have an '=' to set the balance to a given value.

• Balances use 4 decimals, balance movements use 2. It may be interesting to make them equal :thinking:

[Kaian]

Balances use 4 decimals, balance movements use 2. It may be interesting to make them equal