Show the time when a poll starts and ends

This is consistent with the way we show the duration of a budget and its
phases. Since budgets are the section with the most recent changes in
the admin area, we're using it as a reference.

Note that, unlike budgets (which are shown to finish at the beginning of
their ending day), a poll has always been considered to finish at the
end of their ending day, so we're showing it this way.

We're also solving a minor usability issue. While it's pretty intuitive
that a poll starting on a certain date will start at the beginning of
the day, a poll ending on a certain date isn't clear about when it
finishes exactly: is it at the beginning of the day, or at the end of
the day?

So now we're making this point clear.

spec/system/admin/poll/polls_spec.rb

@@ -67,8 +67,8 @@ describe "Admin polls", :admin do
 
     expect(page).to have_content "Poll created successfully"
     expect(page).to have_content "Upcoming poll"
-    expect(page).to have_content I18n.l(start_date)
-    expect(page).to have_content I18n.l(end_date)
+    expect(page).to have_content "#{I18n.l(start_date)} 00:00"
+    expect(page).to have_content "#{I18n.l(end_date)} 23:59"
 
     visit poll_path(id: "upcoming-poll")
 
@@ -93,7 +93,7 @@ describe "Admin polls", :admin do
 
     expect(page).to have_content "Poll updated successfully"
     expect(page).to have_content "Next Poll"
-    expect(page).to have_content I18n.l(end_date.to_date)
+    expect(page).to have_content "#{I18n.l(end_date)} 23:59"
   end
 
   scenario "Edit from index" do