Add more alt attributes

[nylen]

Follow-up to #368. That PR added alt attributes to the <img class="avatar" tags for each comment's author, to improve accessibility for screen readers and other assistive technologies. This PR adds alt attributes to a couple of remaining places (the "Participants" list at the bottom of the comment section, and user avatars that appear in quoted posts):

Credit @timbocode for the first commit and the image. Link to this content (with this PR already applied): https://www.classicpress.net/blog/how-to-read-the-wp-config-file-and-what-can-i-do-with-it/