ETT-1542: fix viewability table viewport a11y issue

[carylwyatt]

Most of this is reformatting the html, but I also removed unnecessary comments and whitespace.

While working on the css for this, I realized that the span that houses the enumchron data (<span class="IndItem">) was being rendered even if there isn't enumchron info for that volume, so I added a conditional to take care of that unnecessary markup that was also adding whitespace to the table.

Gayathri has approved the UI and a11y for this, so this PR is a second set of eyes of markup and php. This is staged on dev-3 if you want to look: https://dev-3.catalog.hathitrust.org/Record/000140237

[eumalin]

This check treats the text "0" as if it were empty.

{if (isset($ld.enumchron) and $ld.enumchron)} will hide the value when $ld.enumchron is the string "0", because PHP and Smarty treat "0" as false in a boolean check. Before this PR, the value was always shown, even if it was "0".

If a volume or issue is ever labeled just "0", it will now disappear from the link instead of showing up.

Suggested fix: check for an empty string directly instead of relying on truthiness, for example:

{if isset($ld.enumchron) and $ld.enumchron !== ''}

This same check is repeated on other lines, so the fix should be made in all five places (or pulled out into one shared check before the if/elseif chain).

[carylwyatt]

@eumalin Thanks for the feedback! I googled a bit and found this {capture} function in smarty that holds template pieces as variables, so I used that to compute the enumchron template piece one time if it exists for that record and then insert where needed.
Edit to add: re-staged on dev-3

[eumalin]

Looks good, thank you!