Added IS_DEMO flag to simplify demo updating

app/admin/2fa/edit_user.php

app/admin/2fa/save.php

app/admin/api/edit-result.php

app/admin/authentication-methods/edit-result.php

app/admin/groups/add-users-result.php

app/admin/groups/edit-group-result.php

app/admin/groups/remove-users-result.php

app/admin/languages/edit-result.php

app/admin/mail/edit.php

app/admin/mail/test-mail.php

app/admin/password-policy/save.php

app/admin/powerDNS/defaults-save.php

app/admin/powerDNS/settings-save.php

app/admin/scan-agents/edit-result.php

app/admin/sections/edit-result.php

app/admin/settings/settings-save.php

app/admin/users/edit-result.php

app/admin/verify-database/fix.php

app/admin/verify-database/index.php

app/dashboard/index.php