New header style & started the responsive styling

Updated the style of the header to make it more organised and provide a
better layout for all screen sizes. I also have started to make the
website more responsive by implementing a grid system and screen
breakpoints.

* Change the style of the header for a cleaner look
* Added a grid system (unsematic)
* Added a mobile navigation that shows when on small mobile like screens
* The banner changes sizes when on tablet sized screens.
* The three unique sections are now in a grid of three.