Handle permissions for ExternalIssue instances

This fixes the remainder of the GitPushService specs.

app/models/ability.rb

@@ -9,6 +9,7 @@ class Ability
       when CommitStatus then commit_status_abilities(user, subject)
       when Project then project_abilities(user, subject)
       when Issue then issue_abilities(user, subject)
+      when ExternalIssue then external_issue_abilities(user, subject)
       when Note then note_abilities(user, subject)
       when ProjectSnippet then project_snippet_abilities(user, subject)
       when PersonalSnippet then personal_snippet_abilities(user, subject)
@@ -424,6 +425,10 @@ class Ability
       end
     end
 
+    def external_issue_abilities(user, subject)
+      project_abilities(user, subject.project)
+    end
+
     private
 
     def named_abilities(name)

spec/services/git_push_service_spec.rb

@@ -367,7 +367,7 @@ describe GitPushService, services: true do
             }
           }.to_json
 
-          execute_service(project, user, @oldrev, @newrev, @ref )
+          execute_service(project, commit_author, @oldrev, @newrev, @ref )
           expect(WebMock).to have_requested(:post, jira_api_transition_url).with(
             body: transition_body
           ).once
@@ -378,7 +378,7 @@ describe GitPushService, services: true do
             body: "Issue solved with [#{closing_commit.id}|http://localhost/#{project.path_with_namespace}/commit/#{closing_commit.id}]."
           }.to_json
 
-          execute_service(project, user, @oldrev, @newrev, @ref )
+          execute_service(project, commit_author, @oldrev, @newrev, @ref )
           expect(WebMock).to have_requested(:post, jira_api_comment_url).with(
             body: comment_body
           ).once