dmd: Make the first row/column of dmddetails table sticky

When dmddetails has many rows, it can be easy to lose track of what each column represents. Similarly, since the table is wide, one can lose track of what package a row corresponds to when scrolling to columns on the right.

In order to keep them visible, a few more css rules are needed.

To keep the headers visible, the entire thead, rather than just the th, needs to be sticky to the top of the viewport. This ensures both rows of the table header are visible.

To keep the first column visible, change the first column in the tbody to a th and mark it sticky to the left of the viewport. Since debian.css makes th bold, also set these th back to normal font-weight.

In addition, the th in the first row of the table head needs to be sticky to the left. We can't simply select the first th of the table head because there are two rows and a rowspan involved.

Closes: #1075725