Migrate Bootstrap dropdown to GitLab UI GlDropdown in ee/app/assets/javascripts/epic/components/epic_create.vue

WARNING: this issue relates to a non-standard drop-down:

The following vue component contains a bootstrap data-toggle="dropdown" attribute and should be replaced with our new GlDropdown component:

• ee/app/assets/javascripts/epic/components/epic_create.vue

Instructions

• Replace uses of data-toggle="dropdown" with GitLab UI components <gl-dropdown>, <gl-dropdown-divider>, <gl-dropdown-header>, and <gl-dropdown-item>.
• Take before/after screenshots of each button in the file, and include them in the Merge Request description.
• Request review from a designer within your stage group. If you are outside of a stage group, ping a UX foundations member.

GlDropdown documentation can be found here: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-dropdown--default

Part of &4134