Remove CSS bootstrap dependency

[GuillaumeGomez]

Follow-up of #15208.

This PR removes the CSS bootstrap dependency which contains 1504 CSS rules and weights 16.1 kB minified. Considering we used less than 50 of these rules, it's quite a waste.

So this time, there are minor UI changes:

• The "expand/collapse all" buttons icon changed. It now uses the one from font-awesome. It's quite close but not exactly the same.
• The layout is slightly different (you need to switch between two tabs to actually see the difference).

Before this PR:

With this PR:

With this, we will be able to go even further on reducing the page size next. =D

This time, the DOM size itself reduced a bit but the difference is too small to be noteworthy.

r? @samueltardieu

changelog: Remove CSS bootstrap dependency

[samueltardieu]

Since this is user-facing, I'll let @flip1995 weight in here.
r? @flip1995