webadmin: rename the column 'Files Rebalanced' to 'Files Migrated' in remove-brick status

Rename the column 'Files Rebalanced' in remove-brick status pop to 'Files Migrated'.
New RemoveBrickStatusPopupView was implemted by inheritng from RebalancePopUpView.

Change-Id: I7675c16d6a96adc09803cf81be6a776f71500501
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1024606
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>