Migrate "import_actions_cell.vue" to GlDisclosure

What does this MR do and why?

Migrates app/assets/javascripts/import_entities/import_groups/components/import_actions_cell.vue from GlDropdown to GlDisclosureDropdown

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• Open "New group" -> Import (/groups/new)
• Use https://gitlab.com as source instance and GeK1Nis4j-SY1X4sqE5c as personal access token (this token is from separate user on GitLab instance with 0 real data available, so we do not expose any security risks here)
• you will see dropdowns

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #413493 (closed)