Add vertical legend to performance dashboard

[Wittiest]

Problem

The Performance Dashboard legend and chart become unreadable if you have greater than ~40 job classes

Solution Description

• Introduces a custom vertical legend and uses it in the Performance Dashboard chart.
• Does not use the vertical legend anywhere else

Screenshot of Performance Dashboard

Screenshot of Jobs Dashboard

Shows that there was no regression

Solution Reflections

• There may be a better approach across graphs to have a hybrid (horizontal graph for < X items, vertical, scrollable graph for N items)
• It may be worth implementing something like the sidekiq metrics dashboard where the table items have a checkbox that controls what appears on the chart.

Implementation Details

• Added vertical: true plugin option to charts and followed the general guidance in Chart.js HTML Legend to implement a custom legend
• Add a chart_container partial which checks for whether the legend is vertical and modifies the layout

[bensheldon]

@Wittiest wow, thank you for doing this! This is not trivial at all 🙇🏻

I made a few tweaks but I'm satisfied with this. I think it's flexible enough that if we wanted to do more responsive improvements we can do so in the future 👍🏻