Describe the bug

Email addresses are not allowed as user name in the user creation form: UI gives an error "User name cannot contain special characters". Though email-as-user-name is permitted in backend.

Steps to reproduce

1. Launch "Create user" in Administration Settings > Users.
2. Enter email address in "User name" field.

Expected behavior

When entering email as user name in the user creation form it should be accepted without errors.

Actual behavior

When entering email as user name in the user creation form it gets rejected with error "User name cannot contain special characters".

Additional context

Workaround 1. As the username validation is performed only in CreateUserModal.vue, but not performed in EditPanel.vue, my current UI-based workaround is:

1. Create a user without @ in user name, e.g. replacing @ with underscore: sk_domain.tld.
2. Edit the user and put @ back in user name, e.g. sk_domain.tld -> sk@domain.tld.

Workaround 2. Alternatively, auto-provisioning also works with email addresses as user names.

The issue seems to be caused by extra/redundant escaping in https://github.com/owncloud/web/pull/9703/files#diff-e4578202fcddaeb2086e30f2ed4fa01d7fc7944c8d185928937afa26eeb835b9R144