Dropdown action item button styles override GlButton

from: gitlab-org/gitlab-ui!4623 (comment 2135981324)

issues(non-blocking): here are some things that I thought were problematic while taking a close look at the component construction. We might want to work on these asap to prevent any issues.

• The select all button here uses <gl-button> contorted beyond recognition. We might want to untangle that to avoid unexpected results

  

• As pointed out in gitlab-org/gitlab-ui!4623 (comment 2134844100) footer actions also use a heavily modified <gl-button> . We might want to untangle that to avoid unexpected results

• Consider separating out dropdown items that are actions and those which are options selected from a list.