Convert submission monitor chart to ECharts

The TimelinePlot Vue component was originally designed to be the replacement for all other plots across the site, first introduced on the submission monitor page.  Since then, we've decided to use Apache ECharts instead of this D3-based approach.  This PR completely rewrites TimelinePlot.vue with ECharts.  The new plot is functionality equivalent, with a handful of minor UI improvements.

Author: williamjallen