pgAdmin 4 commit: Some browsers don't properly support tri-statecheckb

Some browsers don't properly support tri-state checkboxes, so create our own control to handle true/false/null. Fixes
#2848

Branch
------
master

Details
-------
https://git.postgresql.org/gitweb?p=pgadmin4.git;a=commitdiff;h=930dd8af1fac500749c7a9efa2302b5f3f52b4d4
Author: Murtuza Zabuawala <murtuza.zabuawala@enterprisedb.com>

Modified Files
--------------
web/pgadmin/feature_tests/test_data.json           |  15 +--
web/pgadmin/feature_tests/view_data_dml_queries.py |  16 ++-
web/pgadmin/static/css/bootstrap.overrides.css     |  28 +++++
web/pgadmin/static/js/slickgrid/editors.js         | 127 ++++++++++++++++++---
4 files changed, 157 insertions(+), 29 deletions(-)