refactor: replace is_admin boolean for a role based model (attendee, staff, admin, judge).

[enricoprazeres]

• Replace field :is_admin, :boolean in Ares.Accounts.User for field :role, Ecto.Enum, values: [:attendee, :staff, :admin, :judge].
• The migration must backfill (is_admin: true -> role: :admin, resto -> :attendee).
• Update wherever is_admin is used (accounts.ex, team_changeset, navbar.ex, profile.ex, team-formation.ex, dashboard.ex, event_settings.ex)
• Update seeds

For this issue, is_admin should not be removed.