Meta field validation

[the-frey]

Some fields could end up comically long and break things. Probably worth introducing a sensible maximum for all the string fields.

[the-frey]

For now, resolving on the front-end but it's worth considering.

[the-frey]

Other validation for formatting etc is available on the front-end https://github.com/envoylabs/whoami-ui/blob/main/util/forms.ts

Some of this might be worth porting to the contract - e.g. the URLs stuff, for ex