Summary tab displays even when all EVL handlers are disabled

Problem/Motivation

If one or more EVL handlers are added to a webform, an "Element value limits" summary sub-tab becomes available under the webform's "Results" tab. But, if I disable all EVL handlers for the webform, I would expect this summary sub-tab to go away. Currently it remains and shows the value counts as if all EVL handlers are active.

Steps to reproduce

1. Create a webform.
2. Add an Element values limit handler.
3. See that the "Element value limits" summary sub-tab is available under the webform's "Results" tab.
4. Disable the Element values limit handler.
5. See that the "Element value limits" summary sub-tab is still available under the webform's "Results" tab.

Proposed resolution

- If no EVL handlers are enabled, we should hide this tab.
- The page should only show summary tables for enabled EVL handlers.

Remaining tasks

Patch + review.

User interface changes

Disabled EVL handlers will no longer show up in the "Element value limits" summary sub-tab under a webform's "Results" tab. When no EVL handlers are enabled, the sub-tab itself will not show.

API changes

None.

Data model changes

None.