This change fixes a bug where Vikunja would not correctly check if a migration was already running. That meant it was not possible for users who had never before migrated anything to start a migration, because Vikunja assumed they already had a migration running for them.
This state was neither properly reflected in the frontend, which is now fixed as well.
Username and password are validated in the api for length and whitespaces. Previously, the api would tell the user "hey you got it wrong" but the error was not reflected properly in the UI. This change implements a client-side validation which mirrors the one from the api, allowing instant validation and better error UX.
