Ensure landmarks have a label

[mxdvl]

We should ensure that our <aside> elements have a label.

Follow-up on #5036

Multiple landmarks

If there is more than one complementary landmark role or

element in a document, provide a label for each landmark using the aria-label attribute, or, if the aside has an appropriately descriptive title, point to it with the aria-labelledby attribute. This label will allow an assistive technology user to be able to quickly understand the purpose of each landmark.

<aside aria-label="Note about usage">
  <!-- content -->
</aside>

…

<aside id="sidebar" aria-label="Sponsors">
  <!-- content -->
</aside>