Allow seeding a specific tenant with db:dev_seed

Until now, running `db:dev_seed` created development data for the
default tenant but it was impossible to create this data for other
tenants.

Now the tenant can be provided as a parameter.

Note that, in order to be able to execute this task twice while running
the tests, we need to use `load` instead of `require_relative`, since
`require_relative` doesn't load the file again if it's already loaded.

Also note that having two optional parameters in a rake task results in
a cumbersome syntax to execute it. To avoid this, we're simply removing
the `print_log` parameter, which was used mainly for the test
environment. Now we use a different logic to get the same result.

From now on it won't be possible to pass the option to avoid the log in
the development environment. I don't know a developer who's ever used
this option, though, and it can always be done using `> /dev/null`.

spec/lib/tasks/dev_seed_spec.rb

@@ -1,11 +1,18 @@
 require "rails_helper"
 
 describe "rake db:dev_seed" do
-  let :run_rake_task do
-    Rake.application.invoke_task("db:dev_seed[avoid_log]")
-  end
+  before { Rake::Task["db:dev_seed"].reenable }
 
   it "seeds the database without errors" do
-    expect { run_rake_task }.not_to raise_error
+    expect { Rake.application.invoke_task("db:dev_seed") }.not_to raise_error
+  end
+
+  it "can seed a tenant" do
+    create(:tenant, schema: "democracy")
+
+    Rake.application.invoke_task("db:dev_seed[democracy]")
+
+    expect(Debate.count).to eq 0
+    Tenant.switch("democracy") { expect(Debate.count).not_to eq 0 }
   end
 end