Use 'GlButton' in place of button within Epic app

What does this MR do?

This MR updates ee/app/assets/javascripts/epic/ to use GlButton component from gitlab-ui. This MR is follow-up on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9361.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer