Improve model validations

[rien]

Validations on models are working, but they are ugly when shown to users.

• The UI should show these values in a clear and actionable way
• We should make effort not to expose certain values (i.e. passwords) when validations fail

[redfast00]

When signing up as a new user, users (especially on mobile) sometimes start their username with a capital letter, and then get shown the following error message. It would be nice if at least the regex were shown so they know what they did wrong.