Changes include:
1) Remove underscore-string and sprintf-js packages as we were using only %s. Instead, added a function to do the same. Also changed gettext to behave like sprintf directly.
2) backgrid.sizeable.columns was not used anywhere, removed. @babel/polyfill is deprecated, replaced it with core-js.
3) Moved few css to make sure they get minified and bundled.
4) Added Flask-Compress to send static files as compressed gzip. This will reduce network traffic and improve initial load time for pgAdmin.
5) Split few JS files to make code reusable.
6) Lazy load few modules like leaflet, wkx is required only if geometry viewer is opened. snapsvg loaded only when explain plan is executed. This will improve sqleditor initial opening time.
Reviewed By: Khushboo Vashi
Fixes#4701
