Make WebSection seeds idempotent

Each time we were loading the seeds file, more web sections were
created.

On production it meant running `db:seeds` several times created the
records even if they already existed.

During the test, it meant 5 more records were created before every test,
so after ending a test run, thousands of records existed, making banner
tests very slow.

db/seeds.rb

@@ -9,11 +9,11 @@ end
 
 Setting.reset_defaults
 
-WebSection.create(name: "homepage")
-WebSection.create(name: "debates")
-WebSection.create(name: "proposals")
-WebSection.create(name: "budgets")
-WebSection.create(name: "help_page")
+WebSection.where(name: "homepage").first_or_create
+WebSection.where(name: "debates").first_or_create
+WebSection.where(name: "proposals").first_or_create
+WebSection.where(name: "budgets").first_or_create
+WebSection.where(name: "help_page").first_or_create
 
 # Default custom pages
 load Rails.root.join("db", "pages.rb")