Draft: MR histogram diff handler

Supersedes !191 (closed)

• in the case of daily/weekly test comparisons publish some static html somewhere.
• how do we find the last result, ie reference (e.g. for the weekly).
• store a json in the PR database with the names of histograms which are different. The dashboard can then use that to filter out plots with a toggle.

/cc @sklaver @msaur @dpopov