[DIFE-266] Mark account fields as readonly

TIL input fields marked as disabled are not sent during the request. We used this in the account page to show the user Name, nickname and email. Users shouldn't be able to change those fields. But since form expects these values anyway (even if they're discarded later). This causes problems when trying to update the user About and Personal URL fields.

Instead of using the disabled attribute on form inputs, this MR uses readonly to achieve the same effect but without breaking the form. This MR adds specs ensuring the fields are non-readable, and ensure that the user can still update the About and PErsonal URL fields.