accounts: admin user fixture addition
accounts: admin user fixture addition

* NOTE Moves admin user creation from legacy extension to accounts
  fixture. It resolves a problem with next value of the serial id
  on PostgreSQL. (closes #3216)

Signed-off-by: Leonardo Rossi <5460ec99b71a72e7693c4088b5c2585e76e1c857@cern.ch>

inspirehep/invenio-accounts,tiborsimko/invenio-accounts,inspirehep/invenio-accounts,inveniosoftware/invenio-accounts,tiborsimko/invenio-accounts,inspirehep/invenio-accounts,tiborsimko/invenio-accounts,inveniosoftware/invenio-accounts,inveniosoftware/invenio-accounts