Update admin styles

• make round edges of panels only on top and not bottom

• update panels to hide overflow to not display squared bottom of heading when collapsed

• update link color in heading panel to panel heading color and set color darker for better contrast

• update inactive tabs to contain some rounded borders applied on active tab when moused over and apply dark grey background

• update widget browser to use flex display rules so container on right is always as high as the left hand menu and items within align better then old float column layout