Fix up the opt-groups on the blog landing page

[haydngreatnews]

Associated Issue(s): #

Changes in this PR

• Rebuild the optgroup logic for the blog filtering

Notes

• ifchanged acts on the content of the tag, unless a variable is supplied
  • this means that in the "before" state, the optgroups were being opened, but never closed, as the content of the ifchanged never changed.

Reviewer Checklist

• Check 1
• Check 2

Before view:

Before, html:

                  <optgroup label="2025">
                     <option data-href="/blog/2025/06/" >June</option>
                    </optgroup>
                     <option data-href="/blog/2025/05/" >May</option>
                     <option data-href="/blog/2025/03/" >March</option>
                     <option data-href="/blog/2025/02/" >February</option>
                     <option data-href="/blog/2025/01/" >January</option>
                   <optgroup label="2024">
                    <option data-href="/blog/2024/11/" >November</option>

After view:

After HTML:

<optgroup label="2024">
  <option data-href="/blog/2024/08/" selected>August</option>
  <option data-href="/blog/2024/07/" >July</option>
  <option data-href="/blog/2024/06/" >June</option>
  <option data-href="/blog/2024/05/" >May</option>
  <option data-href="/blog/2024/02/" >February</option>
  <option data-href="/blog/2024/01/" >January</option>
</optgroup>
<optgroup label="2023">
  <option data-href="/blog/2023/12/" >December</option>