pgAdmin 4 commit: Use separate editor for Select2Cell.

Use separate editor for Select2Cell.
This allows us to use the Backgrid key navigation as expected.

This has also resulted into an issue - related unresponsive scrollbar
issue in the SubNodeControl.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=pgadmin4.git;a=commitdiff;h=f7e4e8bea01074fffd0f6f8fc8570a7518ff9137
Author: Murtuza Zabuawala <murtuza.zabuawala@enterprisedb.com>

Modified Files
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web/pgadmin/static/css/overrides.css               |   5 +
web/pgadmin/static/js/backgrid/backgrid.pgadmin.js | 145 +++++++++++++++++++--
2 files changed, 142 insertions(+), 8 deletions(-)