[pgAdmin][RM4348] Theme options in pgAdmin and dark theme

Hi Hackers,

Attached is the patch to introduce theme options in pgAdmin.
Changes include:
1) New theme option in preferences - Miscellaneous -> Themes. You can
select the theme from the dropdown. It also has a preview of the theme just
below the dropdown. Note that, a page refresh is needed to apply changes.
On saving, a dialog appears to ask for refresh.
2) You can create your own theme and submit to hackers. README is updated
to help you create a theme. Theme will be available only after bundle.
3) Correction of SASS variables at few places and few other CSS corrections.
4) Added iconfont-webpack-plugin, which will convert all the svg
files(monochrome) used as icons for buttons to font icons. This will allow
us to change the color of icon by using CSS color property.
5) All the .css files will bundle into a separate file now-
pgadmin.style.css. This will help reduce the size of theme css files as CSS
in .css files will not change with the change of SASS variables.
6) "Dark Beta" theme added as a new theme option.

How theme is built ?
Webpack config is changed to achieve it. On running yarn run bundle, code
will read all the directories in web/pgadmin/static/scss/resources which
will be considered as theme dirs, build a separate css for every theme and
then create a file named pgadmin.themes.json which will have all the
information required.

The patch also fixes RM4173, where there some issue with arrow icons in
browser tree.

Kindly review.

--
Thanks and Regards,
Aditya Toshniwal
Sr. Software Engineer | EnterpriseDB India | Pune
"Don't Complain about Heat, Plant a TREE"