BS4 do not provide vendor prefixes scss mixins anymore as it is meant
to be used with the 'autoprefixer' library. As we only support the last
version of every major browsers in Odoo, we took the decision to not
use the library as it was also complex to integrate in Odoo. We decided
to do the library's work by hand if it ever become necessary.
This commit was made because we found a case where it is necessary: flex
display and flex property are not understood by PhantomJS, so lots of
our JS tests failed because some elements were considered (in)visible
while they are in fact not (in an updated browser). This commit should
be reverted once we implement a better alternative to PhantomJS.