This change adds a widget to adjust change table columns in the user's
settings. If no change table columns are adjusted, the server returns an
empty array and the client determines the default columns, stored in the
gr-change-table-behavior.
Columns can get removed/re-ordered by the user but they are limited to
those specified in gr-change-table-behavior. Column preferences persist
across all gr-change-list items (dashboards, searches, etc).
Feature: Issue 4753
Change-Id: I47d5b9f53a95c0c010b04c4495094f188d85e67e