Base access checks on embed buttons instead of filter formats

d.o issue: https://www.drupal.org/node/2362897
@Berdir it works because currently we don't use individual admin permissions, we just have a single `Administer embed buttons` which is used. So, actually do not use any other permissions other than `use`. I could change it to `parent::checkAccess()` but in current situation, it'll never be used anyway.