Web UI Should use Built-In Echarts Time Axis

[andrewbaldwin44]

Proposal

• The webui currently uses the 'category' type for the charts. We should use the built-in 'time' axis type instead, which will give us better looking time intervals, rather than random splits
• Update how chart history recorded is recorded in Locust to be formatted in the way echarts expects for time data ([time, data][] rather than data[])
• Update tests and types

Tested

• Ensure the mark line (between multiple test runs) continues to work as expected
• Ensure a chart from recorded history is rendered properly
• Ensure the HTML report is properly rendered

Screenshots

[cyberw]

Nice! Ok to merge or is there a reason it is in draft?