Convert submission monitor chart to ECharts (#3677)

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