Move mystery number to a constant

This allows us to more easily stub paginates_per in specs, as well as
clarifies a mystery number. (Not that it was too difficult to understand
before, but it's nicer now.)

ee/app/models/vulnerabilities/occurrence.rb

@@ -8,7 +8,9 @@ module Vulnerabilities
 
     self.table_name = "vulnerability_occurrences"
 
-    paginates_per 20
+    OCCURRENCES_PER_PAGE = 20
+
+    paginates_per OCCURRENCES_PER_PAGE
 
     sha_attribute :project_fingerprint
     sha_attribute :location_fingerprint

ee/spec/support/shared_examples/controllers/concerns/vulnerabilities_actions_shared_examples.rb

@@ -46,9 +46,7 @@ shared_examples VulnerabilitiesActions do
       let(:params) { vulnerable_params.merge(page: 2) }
 
       before do
-        class Vulnerabilities::Occurrence
-          paginates_per 2
-        end
+        Vulnerabilities::Occurrence.paginates_per 2
 
         create_list(:vulnerabilities_occurrence, 3, pipelines: [pipeline], project: project)
 
@@ -56,9 +54,7 @@ shared_examples VulnerabilitiesActions do
       end
 
       after do
-        class Vulnerabilities::Occurrence
-          paginates_per 20
-        end
+        Vulnerabilities::Occurrence.paginates_per Vulnerabilities::Occurrence::OCCURRENCES_PER_PAGE
       end
 
       it 'returns the list of vulnerabilities that are on the requested page' do