Fix for invisible top borders in rendered HTML tables

[rich-iannone]

This PR addresses the problem whereby gt tables, when rendered as HTML, have invisible top lines of cell borders (not visible unless 2px or larger).

The problem is isolated to the CSS rules of the gt_row class:

  .gt_row {
    padding: $row_padding; /* row.padding */
    margin: 10px;
    border-bottom-style: solid;
    border-bottom-width: thin;
    border-bottom-color: #D3D3D3;
    vertical-align: middle;
    overflow-x: hidden;
  }

Substituting all border-bottom-* rules for border-top-* avoids the line-drawing conflict between row lines and border lines. That is the only change in this PR and, through several visual tests, the changes made here fix the display issue.

Fixes: #358.

[schloerke]

exibble %>%
  select(num, date, datetime) %>%
  gt() %>%
  tab_style(
    style = cell_borders(),
    locations = cells_data(
      columns = vars(num),
      rows = num > 100 & num < 10000)
  )