Add user to cache dependencies of vote in events index

When fragment caching is enabled—

    --- a/config/environments/test.rb
    +++ b/config/environments/test.rb
    @@ -27 +26,0 @@
    -  config.action_controller.perform_caching = false
    @@ -29 +27,0 @@
    -  config.cache_store = :null_store

—the test of voting fails with:

    expected to find text "Not rated" in "…You voted: 3/5…"
    # ./spec/features/voting_spec.rb:15
    # ./spec/features/voting_spec.rb:13
    # ./spec/features/voting_spec.rb:57

i.e. the vote displayed on the event index is another user’s vote,
because the fragment’s cache dependencies don’t include the current
user.

re #2767

app/views/admin/events/_datatable_row.haml

@@ -1,4 +1,4 @@
-- cache ['admin', conference_id, program, event] do
+- cache ['admin', conference_id, program, event, current_user] do
   %tr{ id: "event-#{event.id}" }
     %td
       = event.id