Support additional numeric formats on asset amount inputs

[aaroncox]

When sending tokens, I often will copy exact values from external sources and paste them into the Unicove send token flow.

Some of those numbers are formatted with a thousands separator (e.g. 1,234.5678), which our current inputs don't recognize as valid amounts. Each time after I paste I need to remove the thousands separator to get it to recognize the value (e.g. 1234.5678).

We could strip out characters we deem invalid, but in the future if we ever try to support alternative thousands/decimal separators (e.g. 1.234,5678 or 1 234,5678) that would cause problems. For reference.