Improve the process of deleting image tags

Problem to solve

It can be difficult to delete image tags from the container registry UI. Following #327693 (comment 632953808), this issue proposes improving the UX for deleting individual tags.

Proposal

• User should be able to delete tags individually in the list/table view.
• The button with a trash bin icon should be replaced by a more actions dropdown button.
  • component:button ellipsis https://design.gitlab.com/components/button/#ellipsis
  • Clicking the more action button displays the Delete tag option.
    • Clicking Delete tag triggers the existing flow for deleting an individual tag.
  • Clicking anywhere outside the dropdown closes it.