In dashboard alerts-table, select ranges of alerts on shift+click.
In dashboard alerts-table, select ranges of alerts on shift+click.

Currently, there's no way to select ranges of alerts.
Fortunately, shift+click doesn't yet do anything special, and is a common way to
afford selecting ranges of items in a list.

This CL listens for click events on checkboxes other than #header-checkbox.

If the user had not yet clicked on any other checkboxes before shift+clicking
one of them, then alerts-table assumes that the user wants to select all
checkboxes starting from the top and going down to the checkbox that was
shift+clicked.

The user does not need to be careful to first click the first checkbox in the
intended range and then shift+click the last checkbox -- the user may click the
last and then shift+click the first.

BUG=catapult:#3569

Review-Url: https://codereview.chromium.org/2904323002

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