[FIX] web: correctly handle 'oe_read|edit_only' in lists

ClassNames 'oe_read_only' (resp. 'oe_edit_only') can be used in
x2many lists to hide columns in 'edit' (resp. 'readonly') modes.

However, before this rev., there were two issues:
 1) Having such a className on a <button> tag didn't work well as
    the 'display: none' rule didn't apply on the body cell (it only
    applied on the button). Same could be observed on <widget>
    nodes.
 2) Having more than one invisible column (with those classNames)
    didn't work either, because the footer cells weren't hidden.
    We didn't notice it with one invisible column, because the
    footer contained one cell less than the header and the body.

Those two issues could be observed on the mrp.bom form view.

Task 2068261