Mark commentary as irrelevant(/not interesting)

[nonamethanks]

Ref topic #16876.

To avoid users deleting what they think is a bad commentary, an ideal way to handle "uninteresting" commentaries would be to mark them in some way that they're not displayed as prominently if they're irrelevant (thinking of something like "Twitter LOG" or the common "無題", untitled, or "rkgk").
This gives us the chance to preserve the commentary while at the same time allowing users to decide whether to keep them or hiding them, which is better than losing it altogether.
It would also allow us to backfetch all existing posts' commentary without causing civil war on the forums, since some users actively remove commentary they don't like from the upload page.

The first thing I had in mind was something like replacing the commentary field with the "expand" function of DText with a header "Show commentary marked as <insert term to denote it's interesting idk>" when an user marks the commentary as hidden. The button for hiding it can also simply be added to the already existing commentary form.