Support PieFed's "extra fields" in profile

[Sjmarf]

PieFed allows you to add up to 4 key-value pairs to your profile, which is useful for things like Pronouns and location.

To discuss: how do we want to display these? Most of the time, I'm guessing users will be using a small selection of keys ("pronouns" etc). Do we want to localize that?

[Sjmarf]

Also worth considering whether #2123 would link into this system

[sunshinehot]

Maybe just show one field then have a plus icon in the bar to add more.