Dark mode should persist locally on browser #1

[NickolasBenakis]

Hello Stefanos,

The following pr solves the following issues:

1. prettier configuration file for code formatting
2. made reducer feel more like es6 with spread operators, switch case
3. persistence of theme using session storage and mount lifecycle, ** tell me if you would like another technique
4. React.memo for Header component which gets unnecessary updates on every keystroke.

[NickolasBenakis]

Τι θα γινει φιλε; Θα το μερτζαρουμε το εργαλειο;