Check HTML validity of all pages

[Alexa Cain]

Go through each page and verify that all HTML is valid. There were several instances of broken tags (that have already been fixed); further testing is in progress.

[Alexa Cain (caina1)]

HTML form on List settings page is marked as invalid, as is the following div.

Steps:
Select Lists
Select a list
Select "Settings" from the list tool bar (with "Info" "Members" "Mass Subscribe" etc., NOT the main Postorius toolbar on the top of the page)
View page source.

[Terri (terriko)]

I think with the ghc12 changes committed, this is fixed for now. No doubt we'll want to do another check before release, but we can open another bug if problems are found.