Update screenshots

added Good first issue Help wanted T: feature labels

assigned to @c1e0

@akwizgran Do you mean the screenshots on the user manual or Play Store?

Oh, I see I'm on the website repo. But there aren't any screenshots on the website, but for the gif at https://briarproject.org/, which looks updated to me.

The gif shows the old chat bubbles, although that's not a big deal. I think the Play Store screenshots were what I had in mind when I opened this.