Add read-only date of birth field to attributes view form

It works and looks pretty nice, I think. Whether the code is sensible, I have no idea. It really felt like no matter how I did this, it would be a little bit of a hack, unless I fundamentally change some basic structure, which I wouldn't feel comfortable doing without some pointers/suggestions. :)

To me, it would've felt strange to make some new class or generic solution for this, though, since the whole birthday attribute thing is already kind of a hack that has its own special setting, like it's already an exception case, so I might as well just modify the exception case