Set unique modal IDs

What does this MR do?

This fixes a bug where trying to delete a repository from the image registry caused multiple confirmation modals to open when there are multiple repositories. This is due to the fact that each repository has one dedicated modal but they all have the same ID.

modal_bug

• Introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29505
• Reported in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29505#note_189882312

To fix this, we simply generate a unique ID for each modal to prevent conflicts.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style guides

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

/cc @costlockerbot