feat: Changed username max_length to the specified by django
Changed USERNAME_MAX_LENGTH to the max specified by Django.
This PR changes the maximum length for usernames defined using [USERNAME_MAX_LENGTH](https://github.com/edx/edx-platform/blob/ef77ddc18db5a8cbe2f90efe18b0386649368c1b/openedx/core/djangoapps/user_api/accounts/__init__.py#L18) in the accounts constants, it was changed from 30 chars to 150 chars following the max stated by the Django documentation.
This change affects the user creation, given that a username with more than 30 chars won't be cut off or rejected, instead, this will happen only with usernames with more than 150 chars.
Username max length stated by Django docs: https://docs.djangoproject.com/en/2.2/ref/contrib/auth/#django.contrib.auth.models.User.username
1. Go to /register
2. Fill the registration form with a username larger than 30 thars, e.g, usernameusernameusernameusername
3. The registration must end successfully and if you check the user, it will have the same username defined above.
4. If you register a username with length greater than 150 chars, the username will be cut off.
Commit that sets USERNAME_MAX_LENGTH to 30 chars: https://github.com/edx/edx-platform/commit/6976a33a8521d0fc8089d418a6a5efb1e7915d89