[pgAdmin4][RM2927] Move all CSS into SCSS files for consistency and ease of colour maintenance etc.

Hi Hacker,

Attached is the patch to replace CSS with SCSS wherever needed. Highlights
of patch:
1) Use of globally defined colors and styles across the complete
application using SCSS.
2) Modularity in CSS - All the SCSS and CSS files in different modules will
be pulled for bundling automatically.
3) Remove hard coded hash codes in different JS files and use CSS class
instead.
4) Replace different out of the theme colors with colors defined in global
SCSS variables.
5) Removed '~bootstrap/dist/css/bootstrap-theme.css'. We dont need it
actually. This will bring default bootstrap design with less gradient use
(gradient kills performance)

Please note, there will be slight color differences from the original. I
have tried to cover most of the app but may have missed few, please point
out.
Also, it is mandatory for wcDocker to provide a theme. To stop it, we need
to change wcDocker code. So for now I have provided empty css file.

Kindly review.
--
Thanks and Regards,
Aditya Toshniwal
Software Engineer | EnterpriseDB Software Solutions | Pune
"Don't Complain about Heat, Plant a tree"