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. 
TBH CPUs are a bit unfamiliar ground to me, so I just followed the comments in the requirements doc, but I can follow the top.py script logic and make sense out of it