feat: add user-friendly validation to signup editor

[PurkkaKoodari]

Adds per-field validation feedback to updateSignup and shows it to the user with localization. All validation is done on the server side.

Email validation uses validator which is internally used by sequelize already.

[MikaelSiidorow]

There's a lot of manual validation logic now for the form fields (esp. updateSignup), I guess some of this could be derived from a schema or a validation library 🤔

Awesome features! 🚀

[PurkkaKoodari]

There's a lot of manual validation logic now for the form fields (esp. updateSignup), I guess some of this could be derived from a schema or a validation library 🤔

I don't disagree, though we'd have to build a very custom schema anyway as most of the schema is part of the answers validation. Will definitely use something better for #32

[PurkkaKoodari]

Of course, this will need duplicating into Tietokilta/web for us to benefit