[pgAdmin4][Patch][RM_2400]: Columns with defaults set to NULL when removing contents after pasting in the edit grid

Hi Dave,

*Implementation:*

1) Took a flag 'is_row_copied' for each copied row:

- to distinguish it from add/update row.
- to write code specific to copied row only as it requires different logic
than add row.

2) After pasting an existing row:

- If a user clear the cell value, it must set cell to [default] value if
default value is explicitly given for column while creating table otherwise
[null].

- Again, if a user entered a value in same cell, then removes that value,
set it to [null] (the same behaviour is for add row).

3) Took a 2-dimensional array "grid.copied_rows" to keep track the changes
of each row's cell so that changes made to each cell are independent and
removed once data is saved.

4) On pasting a row, the cell must be highlighted with light green colour,
so triggers an addNewRow event. Now copied row will add to grid instead of
re-rendering all rows again.

5) Moved the common logic into functions.

This patch pass the feature test cases and jasmine test case.
Also I will add the test cases for copy row and send an updated patch.

Please find attached patch and review.

Thanks,
Surinder Kumar