This adds a toggle on the Dashboard datagrid to show or hide archived review requests, similar to that of the All Review Requests datagrid for showing and hiding closed review requests.

a new template block is loaded in the dashboard to expose the toggle button
when the pressed, a show-archived query is appended to the existing URL query
show-archived=0 filters the archived review requests (those that are archived or muted)
show-archived=1 includes all review requests, regardless of their state of visibility

NB: When show-archived is not specified, its default value of is 0.

Manual tests: Archived and muted several review requests. In the dashboard, they disappeared when setting show-archived=0 and reappeared when setting show-archived=1
Unit tests: Added to reviewboard.datagrids.tests to test if archived review requests would appear in the datagrid for a particular user for show-archived=0 and show-archived=1. Also tested for a different user who had not archived any of the same review requests. All tests passed.