Add Custom Reports to reviewboard

Add new reports to Dashboard that provide information that the system
doesnt provide right now. The new reports available are
i) Requests not Closed but with ShipIt
ii) Requests not reviewed by anyone
iii) Request not reviewed by me

The plan is to add more reports and provide an administration url that allows
users to pick and choose which reports they want to see in their installations'
dashboard. It is also planned to provide Aggregate reports .
Added support for various reports.

Reports currently available are
i) Filter review requests by repository (lists only repos that you have access to)
ii) Review requests not reviewed by anyone
iii) Review requests not reviewed by me
iv) Review requests that have Ship-it but not closed.
Add support for selecting reports in dashboard

Provide a Configure menu in Extensions admin that allows a site administrator
to select which of the reports all users can see. Right now, its an all or
nothing, so its not specific to any users or user groups, but will bring it
in later if needed. The report dict is used to construct the names of the
reports so it dynamically expands depending on the number of reports that
get added to the system.

The settings are stored as part of registeredextension table and will be
available in the extension to display only appropriate reports.