Fix #26. Change Exchange Rates API & Refactor code

[Nishith-Savla]

• Replace existing Exchange Rates API with exchangerate.host API
• Replace useEffect with a callback function to reduce re-renders
• Refactor code & components

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[codemaniac-sahil]

Well done @Nishith-Savla

[Nishith-Savla]

Thank you @codemaniac-sahil