Do not remove issues/comments when deleting users

app/models/user.rb

@@ -70,13 +70,13 @@ class User < ActiveRecord::Base
   has_many :snippets,                 dependent: :destroy, foreign_key: :author_id, class_name: "Snippet"
   has_many :project_members,          dependent: :destroy, class_name: 'ProjectMember'
-  has_many :issues,                   dependent: :destroy, foreign_key: :author_id
-  has_many :notes,                    dependent: :destroy, foreign_key: :author_id
+  has_many :issues,                   foreign_key: :author_id
+  has_many :notes,                    foreign_key: :author_id
   has_many :merge_requests,           dependent: :destroy, foreign_key: :author_id
   has_many :events,                   dependent: :destroy, foreign_key: :author_id,   class_name: "Event"
   has_many :subscriptions,            dependent: :destroy
   has_many :recent_events, -> { order "id DESC" }, foreign_key: :author_id,   class_name: "Event"
-  has_many :assigned_issues,          dependent: :destroy, foreign_key: :assignee_id, class_name: "Issue"
+  has_many :assigned_issues,          foreign_key: :assignee_id, class_name: "Issue"
   has_many :assigned_merge_requests,  dependent: :destroy, foreign_key: :assignee_id, class_name: "MergeRequest"
   has_many :oauth_applications, class_name: 'Doorkeeper::Application', as: :owner, dependent: :destroy
   has_one  :abuse_report,             dependent: :destroy

app/views/projects/notes/_note.html.haml

-- return unless note.author
+- note.author = User.new unless note.author
 - return if note.cross_reference_not_visible_for?(current_user)
 - note_editable = note_editable?(note)
 %li.timeline-entry{ id: dom_id(note), class: ["note", "note-row-#{note.id}", ('system-note' if note.system)], data: {author_id: note.author.id, editable: note_editable} }
   .timeline-entry-inner
-    .timeline-icon
-      %a{href: user_path(note.author)}
-        = image_tag avatar_icon(note.author), alt: '', class: 'avatar s40'
+    - if note.author.valid?
+      .timeline-icon
+        %a{href: user_path(note.author)}
+          = image_tag avatar_icon(note.author), alt: '', class: 'avatar s40'
     .timeline-content
       .note-header
-        = link_to_member(note.project, note.author, avatar: false)
+        - if note.author.valid?
+          = link_to_member(note.project, note.author, avatar: false)
         .inline.note-headline-light
           = note.author.to_reference
           - unless note.system