Change "no surge" icon to left-arrow

The ellipsis looked too much like a hyphen, which we use to signify no
data. This fits better with the up/down arrows.

https://www.pivotaltracker.com/story/show/83047320 [Delivers #83047320]
Merge pull request #48 from sealevelresearch/change-no-delta-icon

Change "no surge" icon to left-arrow
Round down "now" to nearest 10 minute increment

This makes it cleaner to look at the times (since we use a 10-minute
interval) as well as giving a better chance of the selected row having
realtime data.

Also improves cache-ability which is a win.

https://www.pivotaltracker.com/story/show/83325086 [Delivers #83325086]
Merge pull request #49 from sealevelresearch/round-down-10-minutes

Round down "now" to nearest 10 minute increment
Shrink table headers on mobile displays