[UI] Replace vue-bar-graph with chart.js

- Backport of #4571
- The usage of the `vue-bar-graph` is complicated, because of the `GSAP`
dependency they pull in, the dependency uses a non-free license.
- The code is rewritten to use the `chart.js` library, which is already
used to draw other charts in the activity tab. Due to the limitation of
`chart.js`, we have to create a plugin in order to have images as labels
and do click handling for those images.
- The chart isn't the same as the previous one, once again simply due to
how `chart.js` works, the amount of commits isn't drawn anymore in the
bar, you instead have to hover over it or look at the y-axis.
- Resolves #4569

(cherry picked from commit a83002679d)

web_src/css/repo.css

@@ -2995,3 +2995,7 @@ tbody.commit-list {
   font-size: inherit;
   line-height: inherit;
 }
+
+#repo-activity-top-authors-chart {
+  height: 150px; /* Pre-allocate the height that will be taken up by the chart, to avoid the container 'jumping'. */
+}