Updated iOS app with categories manton.org

@manton Is this also for < 280 chars posts? I was under the impression that categories were for 280+

@pratik It's for any length of post, but the idea for now is that the category options are hidden when writing short posts on the web because I don't want to complicate the posting UI.

@pratik On the web, yes, first the title field appears, and then as soon as you type something in, the categories appear. We may tweak this. On iOS/macOS, you can pick the categories at any time so they don't automatically appear.