Allow possiblity to change graphs freshness by changing frequency of polling

Hi Canary team,
By using your cool GUI, I had trouble with graphs freshness because the polling frequency was too high and was raising Django limits "OverLimit: This request was rate-limited. (HTTP 413)"

Consequently, I propose a quick patch adding possibility to dynamically change the frequency of the refreshs on the GUI. 
The idea is to store the Object value of the setInterval, plus the graph settings, and stopping/starting a new setInterval with the desired value.

Please excuse the visual I did, as I had no time to specifically look at the DOM for integrating properly.