feat: add caldav tokens (#1065)

# Description

This PR adds API routes to create and manage caldav tokens. These tokens can be used instead of a user password - required for users who are using external auth providers and don't have a password.

Fixes #842

Frontend: https://kolaente.dev/vikunja/frontend/pulls/1186

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1065

pkg/user/user.go

@@ -324,7 +324,7 @@ func CheckUserCredentials(s *xorm.Session, u *Login) (*User, error) {
 		if IsErrWrongUsernameOrPassword(err) {
 			handleFailedPassword(user)
 		}
-		return nil, err
+		return user, err
 	}
 
 	return user, nil