Update to bootstrap 4.1.3

What does this MR do?

Update bootstrap to 4.1.3. We previously had problems with one of their selectors (https://gitlab.com/gitlab-org/gitlab-ce/issues/51362), but I don't think they are going to remove it again from their CSS (as per: https://github.com/twbs/bootstrap/issues/26878)

Bootstrap < 4.1.2 is vulnerable to several XSS attacks, so I thought it might make sense to update and "revert the broken CSS selector" on our site.

Without the CSS fix (download icon)	With the CSS fix

What are the relevant issue numbers?

• Closes: https://gitlab.com/gitlab-org/gitlab-ce/issues/51397

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.