[SEND] User should see confirmation UI after deleting all files

[radishmouse]

After completing delete-all-files flow from #695 , user should see UI indicating that all files were deleted.

Currently, the user is taken back to dashboard.

They can see that their storage is back down to 0B, but there's no other sign that the operation was successful

[thundernotion]

Issues with a parent cannot be on the Milestones board, removing. Either move your sub-issues up to the parent issue, or turn this into an independent Milestone issue that has sub-issues.