[tools] Revamp callstats.html

- Fix diff coloring when a baseline is selected
- Cleanup UI and use dark-mode by default
- Reuse CSS from the system-analyzer
- Support loading mutliple files by default everywhere
- Use hideable panels from system-analyzer

Change-Id: Iafe271d2f86fbbd10a940b3ac99874dd51f45442
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2721763
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73128}

tools/system-analyzer/index.css

@@ -132,7 +132,7 @@ dd {
   font-weight: 400;
 }
 
-.panel > select{
+.panel > select {
   width: calc(100% + 20px);
   margin: 0 -10px 10px -10px;
 }