Change one-minute precision assumptions in UI

[chrismd]

The auto-refresh button in the Browser UI and the redraw command in the CLI both assume minutes as the unit of time. Several users use sub-minute precision, so we at least need the option of doing sub-minute refreshing.

[chrismd (chrismd)]

One step closer to implementing an auto-refresh that is based on the lowest-common-denominator for the current metrics' precision. I've implemented per-metric context data persistence that I will use in in the future to determine the refresh rate.

[Michael Leinartas (mleinartas)]

Migrated to Github: https://github.com/graphite-project/graphite-web/issues/180