Improve annotation feature in BM tool

The annotation feature in the BM tool is currently very basic in that it only allows for editing of a comment string associated with a benchmark time series (i.e. results for all SHA-1s for a given host/platform/branch/benchmark/metric combination).

This could easily be improved in different ways:

• Interactively classify a time series as "suspended" or "active".
• Interactively classify a change within a time series as "omitted" or "keep". Typically we would classify a change as "omitted" if we know that the change has some irrelevant cause (like the benchmark code itself being modified unintentionally).

This effectively provides a filtering mechanism: A suspended time series will be omitted from certain analyses. An omitted change will not contribute to the change analysis for the benchmark in question.