Create campaings before running campaigns tests

We were defining campaigns with `let`. That meant they weren't created
until these methods were used in the tests.

For the test "Do not track erroneous track_ids", that meant the line
`expect(page).not_to have_content campaign2.name.to_s` wasn't really
testing anything, since before this line is executed, the campaign2
wasn't in the database at all, and so obviously its name wouldn't be on
the stats page.

For the test "Track email templates", it meant we were creating the
campaign2 record after visiting the campaign1 page with the browser.
Creating records in the tests after starting the browser might be the
reason why this test has recenty failed in our CI [1]:

 1) Email campaigns Track email templates
     Failure/Error: ds.add params[:event].titleize, Ahoy::Event.where(
                    name: params[:event]).group_by_day(:time).count
     ActiveRecord::StatementInvalid:
       PG::ProtocolViolation: ERROR:  bind message supplies 0
       parameters, but prepared statement "" requires 1
     # ./app/controllers/admin/api/stats_controller.rb:13:in `show'

Using `let!` to create the campaings before the browser starts improves
the situation.

[1] https://github.com/consul/consul/runs/2952333023

spec/system/campaigns_spec.rb

@@ -1,8 +1,8 @@
 require "rails_helper"
 
 describe "Email campaigns", :admin do
-  let(:campaign1) { create(:campaign) }
-  let(:campaign2) { create(:campaign) }
+  let!(:campaign1) { create(:campaign) }
+  let!(:campaign2) { create(:campaign) }
 
   scenario "Track email templates" do
     3.times { visit root_path(track_id: campaign1.track_id) }