Enforce the maximum of 40 items in a collection.

We're only display 40 now (was 20) so enforce this to avoid confusion.

collections/programming-languages/index.md

@@ -40,7 +40,6 @@ items:
  - purescript/purescript
  - PowerShell/PowerShell
  - rakudo/rakudo
- - SWI-Prolog/swipl-devel
 display_name: Programming languages
 created_by: leereilly
 ---

test/collections_test.rb

@@ -32,6 +32,13 @@ describe "collections" do
         assert_empty invalid_slugs, "Invalid item slugs #{invalid_slugs}"
       end
 
+      it "has valid number of items" do
+        items = items_for_collection(collection)
+        assert (1...MAX_COLLECTION_ITEMS_LENGTH + 1).cover?(items.length),
+               "must have no more than #{MAX_COLLECTION_ITEMS_LENGTH} items " \
+               "(currently #{items.length})"
+      end
+
       it "fails if a repository does not exist or is private" do
         errors = []
 

test/collections_test_helper.rb

@@ -5,6 +5,7 @@ require_relative "./test_helper"
 VALID_COLLECTION_METADATA_KEYS = %w[collection created_by display_name image items].freeze
 REQUIRED_COLLECTION_METADATA_KEYS = %w[items display_name].freeze
 
+MAX_COLLECTION_ITEMS_LENGTH = 40
 MAX_COLLECTION_SLUG_LENGTH = 40
 MAX_COLLECTION_DISPLAY_NAME_LENGTH = 100