by introducing another function (coinbaseAvailability)

Follow up to #8666

Auditors:

Test Plan:
1. Open about:preferences#payments
2. Set the budget to 5 USD
3. Click "Add funds"
4. Make sure the button and the title is displayed on the 1st row
5. Set the budget to other than 5 USD
6. Click "Add funds"
7. Make sure 'coinbaseNotAvailable' is displayed