[pgAdmin4][RM2136] Include "?<intver>" on all routes that are loaded

Hi Hackers,

Attached is the patch for RM2136 which will add "?<ver_param>=<ver>" to all
the static files requested and will help in cache busting on upgrading
pgAdmin4.
<ver_param> name is configurable and is present in
config.py-INTERNAL_VERSION_PARAM.
The value <ver> is taken as APP_VERSION_INT (which is updated with every
release).
Setting INTERNAL_VERSION_PARAM value to blank or None will disable the
version param add.
New parameter INTERNAL_VERSION_EXTN (tuple) is also added to specify the
file extensions for which the version param to be added. I have added a few
for now.
Test cases included.

Kindly review.

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