Threads tab

Added a first implementation of the threads tab.

The tab is available under the /dashboard/threads route. 

The tab contains a table with all of the threads the API returns, ordered by cpu_ms.
The top 10 graphs are also rendered as a chart in another box, and there is a possibility to add/remove threads plots from the charts using checkboxes on the threads table. 
This looks odd. There's an idle thread for every vCPU and the numbering starts from zero. AFAICT, this should use "starts with" logic like scripts/top.py in osv.git does.