Clean up selection summary component

This MR is a refactor that cleans up selection_summary.vue, used on the vulnerability report for the bulk select feature:

It does the following:

1. Remove the status_dropdown.vue component and combine it into selection_summary.vue. All the status dropdown component did was pass props to GlDropdown, so there's no need for a separate component just to do that.

2. Move selection_summary.vue into the vulnerability_reports folder. It's only used on the vulnerability reports, so we should move it to where the other report components are.

3. Remove unnecessary computed properties and clean up template HTML.

4. Disable the status dropdown and cancel button when the mutations are running. Also add a loading spinner to the "Change status" button:

Before	After

Related to #337599 (closed)