Allow user to specify the detail report instance linked to from a summary report instance

PROBLEM: A number of summary report templates provide links to a detail report for "drilldown". However, the summary report user interface does not allow the user to specify which instance of a detail report should be linked to - and currently (as of 4.2.2) links to the instance with the lowest ID (CRM-10927). This does not work well when users have created many different instances of a given detail report - as the drilldown provides "unexpected" results.

SOLUTION: If there is more than one eligible drilldown instance, provide a dropdown and allow the user to select one. Dropdown will include an "empty" (- select - ) option which will results in the current default behavior (pick the instance with the lower instance ID).

Implementation
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NOTE: Deepak - this is a draft implementation. Feel free to modify / simplify if you see a cleaner way to do this.

0. Add new column to report_instance table to store drilldown_id

1. Add a new property to report constructors allowing reports to define a "drilldown report_id". For example, CRM_Report_Form_Contribute_Summary would define this as 'contribute/detail'.

2. When a report instance of a report which has this property defined is loaded, check if there are > 1 report instances where report_instance.report_id = drilldown report_id. If so, include dropdown in Report Settings > Other Settings

"Link to Detail Report" [ - select - ]

Dropdown lists eligible report instance titles.

3. Update Report post process saves the drilldown instance id.

4. Modify getNextUrl method used in alterDisplay() to retrieve the specified drilldown instance id.