[TASK] Add an icon for gold. RT-2046 #finish

Fixes https://ripplelabs.atlassian.net/browse/RT-2046

Also added a color for EUR.
@justmoon We're trying to use variables for colors for consistency.  Could you take a look at src/less/ripple/variables.less and add these colors and give them appropriate variable names?  Then we can use variables instead of hex values in tabs.less. Thanks!