Upgrade to Bootstrap 4.3.1

We should upgrade to bootstrap 4.3.1.

Bootstrap 4.3.1 now implements a HTML Sanitizer for popovers:

Update Bootstrap to 4.3.1 to mitigate CVE-2019-8331.

• CVE-2019-8331 (CVSS 3.0 Score 6.1)
• https://blog.getbootstrap.com/2019/02/13/bootstrap-4-3-1-and-3-4-1/

This leads to some interesting problems, as that their sanitizer is rather strict.

---

As part of this issue, we should address https://gitlab.com/gitlab-org/gitlab-ce/blob/97cb22820d79f7c74cacf78a6dba2355bcf1392f/app/assets/stylesheets/framework/modal.scss#L26-29.