Ability to enable/disable AB testing from the global configuration page

Description

Customers and support have asked for a way to disable AB test processing without uninstalling the NuGet package and re-deploying the application.

Solution
A check box is added to the configuration page. It enables or disables all AB testing processing.
When enabled:

• All normal A/B testing resumes.

When disabled:

• On the Publish Menu, "AB Test changes" option should not be displayed.
• All A/B testing processing (creating views and conversions) is disabled.
• Users can still stop a test or pick a winner.
• Scheduled tests are marked as running at the scheduled time. However, no processing will occur.
• There should be no impact on page load performance.
• On details and pick a winner views, a message indicates that A/B feature is disabled.

The Enabled/Disabled configuration value is preserved and respected after updates and restarts of the web application.
The default state is enabled (that is, AB testing will be enabled by default), including upon upgrade.