Multi metric support for ab tests (using redis)

[sdhull]

This is really a WIP PR because I only implemented the support for redis adapter. I have neither the time nor the inclination to get this working for mongo & AR, though I was careful not to break those adapters.

You can feel free to drag this across the finish line, as I am allowing edits from vanity maintainers. This also may be useful for others who are struggling with similar problems (eg, tracking funnel conversion with experiments).

Improvements:

• track each metric conversion separately for a given ab test [redis only]
• show conversions per metric on the dashboard [redis only]
• show unique & "total" conversion counters on dashboard ("total" previously completely hidden & unused afaict)
• make the dashboard full width
• fixes "total" conversion tracking to respect the implicit param (eg, if you are tracking "dollars added to cart", this is being tracked in current vanity release regardless of whether or not that conversion was done by a participant in your experiment)

[phillbaker]

@sdhull thanks for the PR, it'll take me a couple of days to review, but this is awesome! It's good work toward a solution for https://github.com/assaf/vanity/issues/230.

[bensheldon]

Closing this for inactivity. Please comment/reopen if you'd like to pick it back up.