Improvement to Status pages (Status alerts, Slow running tests)

As specified in #185 (comment 249430), the Status alerts and Slow running tests page could use the improvement of having filters based on suite/architecture as in Pending and Status failing pages.

• convert these pages to be generated dynamically, and not as static HTML (this may imply changing how they work entirely)
• the "Platform-specific issues" page is not really useful at the moment, we can probably drop it.
• modify existing templates to reuse the job listing template when applicable
• replace the "All/latest 50/etc" mechanism with pagination
• add filtering by architecture/suite to all listings