pgAdmin 4 commit: Adding the SQL tab for the nodes, with 'hasSQL' prope

Adding the SQL tab for the nodes, with 'hasSQL' property set to true, in
create/edit mode. It will fetch the modified sql using the 'msql' url
specific to that node.

Also, modified by Ashesh before committing it.
i.e. Added code comments, cleanup code for the control, etc.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=pgadmin4.git;a=commitdiff;h=d56ddb9fa4ad0976333763369726de522f24cfb4
Author: Murtuza Zabuawala 

Modified Files
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.../browser/templates/browser/js/browser.js        |   4 +-
web/pgadmin/browser/templates/browser/js/node.js   |   2 +-
web/pgadmin/static/js/backform.pgadmin.js          | 163 ++++++++++++++++++---
3 files changed, 146 insertions(+), 23 deletions(-)