Improve language of email notifications of MR and Issue reassignment

app/views/notify/reassigned_issue_email.text.erb

-Reassigned Issue <%= @issue.iid %>
-
-<%= url_for([@issue.project.namespace.becomes(Namespace), @issue.project, @issue, { only_path: false }]) %>
-
-Assignee changed <%= "from #{@previous_assignees.map(&:name).to_sentence}" if @previous_assignees.any? -%>
- to <%= "#{@issue.assignees.any? ? @issue.assignee_list : 'Unassigned'}" %>
+Assignee of Issue <%= @issue.iid %> ( <%= url_for([@issue.project.namespace.becomes(Namespace), @issue.project, @issue, { only_path: false }]) %> ) changed <%= "from #{@previous_assignees.map(&:name).to_sentence}" if @previous_assignees.any? -%> to <%= "#{@issue.assignees.any? ? @issue.assignee_list : 'Unassigned'}" %>

app/views/notify/reassigned_merge_request_email.text.erb

-Reassigned Merge Request <%= @merge_request.iid %>
-
-<%= url_for([@merge_request.project.namespace.becomes(Namespace), @merge_request.project, @merge_request, { only_path: false }]) %>
-
-Assignee changed <%= "from #{@previous_assignee.name}" if @previous_assignee -%>
- to <%= "#{@merge_request.assignee_id ? @merge_request.assignee_name : 'Unassigned'}" %>
+Assignee of Merge Request <%= @merge_request.iid %> ( <%= url_for([@merge_request.project.namespace.becomes(Namespace), @merge_request.project, @merge_request, { only_path: false }]) %> ) changed <%= "from #{@previous_assignee.name}" if @previous_assignee -%> to <%= "#{@merge_request.assignee_id ? @merge_request.assignee_name : 'Unassigned'}" %>