DatePicker: Write tests and refactor

The DatePicker component is already created and it's functional, but it was written some time ago and it can be refactored (migrating from class-based to hooks). In order to achieve a good result, we also need a good test coverage.

Running storybook locally

1. Fork and clone the repo
2. Install the necessary dependêncies using npm (run npm i on the root folder)
3. Start storybook locally by running npm start, this will start a local server and opens a tab on your browser automatically
4. Uncomment the code on src/DatePicker/DatePicker.stories.js file in order to make it appear
5. Create a DatePicker.spec.js file on the same folder to write tests

Aditional information

We're using React Testing Library to unit test our components and we have a good test coverage, so you can rely on the existing ones.

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