Allow Tamper plugins to be enabled/disabled

The D7 version allows you to enable/disable Tamper plugin instances. This feature would need to be ported.

I think the following needs to happen to achieve that:

• When adding a Tamper plugin instance, it should by default be enabled.
• When updating an existing Tamper plugin instance, we need to ensure that the 'enabled' setting doesn't get lost.
• On the Tamper listing page, checkboxes need to be added to change the 'enabled' status of each Tamper plugin instance. When clicking the button "Save" at the bottom of the page, these changes need to be saved.