Improve the Environments dropdown behavior on feature flags page
Description
At the moment it's possible to select the same environment string multiple times using the select Environments dropdown. It would be better UX if we handle this case and make the selection process more streamlined.
UX proposal
Leverage the multi-select feature of GlCollapsibleListbox
component: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-new-dropdowns-listbox--custom-list-item
Preliminary technical proposal
Once Migrate GLDropdown to GlCollapsibleListbox for ... (!130662 - merged) is merged, we can update the new_environments_dropdown.vue
component to leverage the multiselect feature of GlCollapsibleListbox
and change strategy.vue
component to work with arrays of selected items instead of managing single additions.