Add median/90/95th pctile to monitors statistic in addition to average

[alexhutko]

(moved over from squacapi_client ticket pnsn/squacapi_client#24)

Especially useful in the case of "over the last 5 measurements, alarm me if more than one measurement was crazy (and don't be biased by using average that contains an outlier)". Front end and back end changes. While we're at it for adding median, why not add 95th pctile (I'm not sure if we have 90th already?).

[kyla-m]

Also #391