Make the merge request notification emails more readable

Emphasize the meaningful informations, and remove the less needed ones:

* Use "!" to identify a merge request
* Write the name of the commented file for diff notes
* Don't show the assignee in the "merged merge request" email

app/views/notify/closed_merge_request_email.html.haml

 %p
-  = "Merge Request #{@merge_request.iid} was closed by #{@updated_by.name}"
-%p
-  != merge_path_description(@merge_request, '→')
-%p
-  Assignee: #{@merge_request.author_name} → #{@merge_request.assignee_name}
-
+  = "Merge Request !#{@merge_request.iid} was closed by #{@updated_by.name}"

app/views/notify/merged_merge_request_email.html.haml

 %p
-  = "Merge Request #{@merge_request.iid} was merged"
-%p
-  != merge_path_description(@merge_request, '→')
-%p
-  Assignee: #{@merge_request.author_name} → #{@merge_request.assignee_name}
-
+  = "Merge Request !#{@merge_request.iid} was merged"

app/views/notify/note_merge_request_email.html.haml

-%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
-    = link_to "New comment", project_merge_request_url(@merge_request.target_project, @merge_request, anchor: "note_#{@note.id}")
-  for Merge Request ##{@merge_request.iid}
+- if @note.diff_file_name
+  %p.details
+    New comment on diff for
+    = link_to @note.diff_file_name, @target_url
+    \:
+
 = render 'note_message'