Fix "Medium severity vulnerability found in bootstrap"

[megoth]

We currently have a vulnerability (as reported by snyk) because of Bootstrap 3.3. The proposed fix is to migrate to Bootstrap 4. We could also wait for a release of 3.4, where there is a fix, but I'm not certain when that comes (if ever).

We only use the CSS of Bootstrap, so this is probably a non-issue. But it is annoying to have Snyk report that it is (and it doesn't look good that it does), so we might want to do it anyway.

[dmitrizagidulin]

+1, let's just migrate to Bootstrap 4.

[timbl]

Any way to just import the css?