Streaming payments amount input minor fixes

[arrenv]

Teamwork task: Streaming payments amount input minor fixes

Steps to reproduce

1. Create a new Streaming payments action.
2. Enter in a non-numeric value into the Amount field.
3. Note the error in the console log.
4. Note the width of the amount input field.
5. Enter in an amount that exceeds the balance in the team.

Expected behaviour

• The amount field should only allow numeral values and not be broken by non-numeral values.
• There should be now error in the console when editing the amount field value. Previously resolved it in this PR https://github.com/JoinColony/colonyCDapp/pull/2757
• There should be no "Not enough tokens" amount validation on streaming payments. This is a rate calculation and has no relation to the available funds in the team.

Actual behaviour

• The amount field width breaks when entering in a non-numeral value.
• Adding an amount value should not cause a console error.
• There is an not enough tokens validation on the amount.

[adam-strzelec]

I was unable to replicate the bug with input. On my device everything works fine.