[pgAdmin4][Patch]: Using editorconfig to maintain consistent coding styles between different IDE's

Hi All,

In pgAdmin4 codebase, the coding styles at many places is inconsistent, in
some files standard coding conventions are followed and in some not.

It is difficult to take care of 'tabs', 'spaces', 'trailing white-spaces',
'max_line_length' while development.

So adding .editorconfig to pgAdmin4 will make development easier and will
help in faster patch review.

Added a `.editorconfig` file where all coding styles followed in pgAdmin4
are added, and most of editors/IDE reads this file and follows the rules
while development.

For PyCharm editor, there is no additional configuration or settings
required. it reads .editorconfig.
For Vim editor - developer has to install plugin available.

List of editors supporting `.editorconfig`
http://editorconfig.org/#download

Also, there is a NPM plugin eclint <https://github.com/jedmao/eclint> available
which fixes files automatically and do other tasks as well, for example:

Running following command on terminal fixes issues related to tabs,
trailing whitespaces in existing files:

./node_modules/eclint/bin/eclint.js fix
pgadmin/static/css/bootstrap.overrides.css

If i missed any rule, please let me know
​ i will add.​

Please find attached patch and review.

Thanks,
Surinder