Clean up payments panel

[maxwellward]

• Removes redundant title for payments panel.
• Changes title of gifted nitro to be explanatory enough to not need an info card.
• Changes "at" to "on" to be grammatically correct.
• Fixes rounding of currency to be a maximum of 2 decimal places.
• Change the symbol references in currencies.json to be the actual symbol as basically all modern browsers can display the normal symbols.
• Currency symbol is now returned from getMostUsedCurrency to clean up the payments panel code a bit.

Closes #33

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[peterhanania]

Thank you!