add text templates to new emails

app/views/notify/closed_issue_email.text.haml

+= "Issue was closed by #{@updated_by.name}"
+
+Issue ##{@issue.id}: #{project_issue_url(@issue.project, @issue)}

app/views/notify/closed_merge_request_email.text.haml

+= "Merge Request #{@merge_request.id} was closed by #{@updated_by.name}"
+
+Merge Request url: #{project_merge_request_url(@merge_request.project, @merge_request)}
+
+Branches: #{@merge_request.source_branch} - #{@merge_request.target_branch}
+
+Author: #{@merge_request.author_name}
+Assignee: #{@merge_request.assignee_name}

app/views/notify/merged_merge_request_email.text.haml

+= "Merge Request #{@merge_request.id} was merged"
+
+Merge Request Url: #{project_merge_request_url(@merge_request.project, @merge_request)}
+
+Branches: #{@merge_request.source_branch} - #{@merge_request.target_branch}
+
+Author: #{@merge_request.author_name}
+Assignee: #{@merge_request.assignee_name}