Use fixed md breakpoint for responsive display table CSS rule

Problem/Motivation

In `bs_bootstrap/sass/base/partials/_table.scss` we are doing

.table-responsive {
  @include media-breakpoint-up('$responsive-break') {
    // For big screens its better that responsive table has display table so
    // it expands to full width.
    display: table;
  }
}

Some client projects are overriding `$responsive-break` to a bigger value which can create unexpected results for the above rule.

Proposed resolution

Use `md` breakpoint instead of `$responsive-break`.