Simplify the default layout of notification emails

* Less margin: the content appears as the text of the message.

* Streamlined footer: everything on one line, with a small separator.

* Zimbra (www.zimbra.com) chokes on HTML tables, and doesn't display
the content of the message. Switching to a simpler layout fixes the
issue.

app/views/layouts/notify.html.haml

@@ -3,17 +3,24 @@
     %meta{content: "text/html; charset=utf-8", "http-equiv" => "Content-Type"}
       %title
         GitLab
+  :css
+    p.details {
+      font-style:italic;
+      color:#777
+    }
+    .footer p {
+      font-size:small;
+      color:#777
+    }
 
   %body
-    %table{align: "left", border: "0", cellpadding: "0", cellspacing: "0", style: "padding: 10px 0;", width: "100%"}
-      %tr
-        %td{align: "left", style: "margin: 0; padding: 10px;"}
-          = yield
-      %tr
-        %td{align: "left", style: "margin: 0; padding: 10px;"}
-          %p{style: "font-size:small;color:#777"}
-            - if @project
-              You're receiving this notification because you are a member of the #{link_to @project.name_with_namespace, project_url(@project)} project team.
-              %br
-            - if @target_url
-              #{link_to "View in GitLab", @target_url}
+    %div.content
+      = yield
+    %div.footer{style: "margin-top: 10px;"}
+      %p
+        \—
+        %br
+        - if @project
+          You're receiving this notification because you are a member of the #{link_to @project.name_with_namespace, project_url(@project)} project team.
+        - if @target_url
+          #{link_to "View in GitLab", @target_url}

app/views/notify/new_merge_request_email.html.haml

-%p
-  = "New Merge Request ##{@merge_request.iid}"
-%p
+%p.details
   != merge_path_description(@merge_request, '→')
 
 - if @merge_request.assignee_id.present?

app/views/notify/note_merge_request_email.html.haml

-%p
+%p.details
   - if @note.for_diff_line?
     = link_to "New comment on diff", diffs_project_merge_request_url(@merge_request.target_project, @merge_request, anchor: "note_#{@note.id}")
   - else