Add issues_enabled column to jira_tracker_data table

What does this MR do?

Add issues_enabled column to jira_tracker_data table.

As part of the &3622 (closed), we will have a Jira issues page.

The latest design, #223787[jira-integration-with-new-section.png] (scroll to the bottom) allow configuring and enabling the Jira issues page. We need a new column to support the Enable Jira issues checkbox.

== 20200703154822 AddIssuesEnabledToJiraTrackerData: migrating ================
-- add_column(:jira_tracker_data, :issues_enabled, :boolean, {:default=>false, :null=>false})
   -> 0.0015s
== 20200703154822 AddIssuesEnabledToJiraTrackerData: migrated (0.0016s) =======

Related to #225706 (closed)