Click on legend offers editing color when not in edit mode

## Expected Behavior
Clicking on a legend item should toggle or highlight the corresponding visualization item.

## Current Behavior
Click a legend item offers to edit the color of the series, which I would only expect in the "Edit" mode of the widget (where it appears, so that's good!).
I think it's inconsistent to allow editing the series color where almost everything else is read-only.

## Possible Solution
Change the legend on-click behavior to toggling the series instead of showing the color editor when the widget is not in edit mode.

## Steps to Reproduce (for bugs)
Obvious from the screenshot.

## Context
Other products often allow toggling or highlighting a series based on the legend item, so Graylog is surprising here.
For consistency, I think we also shouldn't allow editing visualization elements if not in edit mode.

## Your Environment

* Graylog Version: 3.2.1