Admin UI adjustments

I am creating this issue to fix some minor issues on the admin UI:

• The description of the "Enabled provider" field is a copy of the description of the "Retry sending mail on error." checkbox.
• The header of table of providers is shorter than its rows - the colspan must be increased. Also, the column should be called "Operations" as other Drupal's tables.
• Typo on the "SMTP Server Settingssss" fieldset.
• The submit button of the provider's edition form should be called "Save" instead of "Submit".
• When editing the provider's machine name, the textfield appears outside the fieldset.
• smtp_admin_provider_delete_submit() expects a third parameter which is not provided.
• After a provider is deleted, the user is not redirected back to configuration form.