Show placeholder instead of invisible displaynames

[indeyets]

I believe we should do our best effort to show some kind of visible placeholder instead of displayname, which consists of invisible characters.

We could use the list from https://invisible-characters.com/

I suggest we could use White Square U+25A1 character

[clbn]

See also: clbn/freefeed-gamma@149269d

[indeyets]

@clbn thanks. that's another way to do that, but seems a bit too harsh… I think white square might be a bit closer to user's intention. They want to be nameless. This solution keeps both the "nameless" aspect and usability of readers

[clbn]

(What to do after those characters are detected would be another topic for discussion. And I personally don't think the user has the right to be nameless, so falling back to the username is the right thing to do for them.)