Implement issuable sidebar partial

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/shared/issuable/_sidebar.html.haml

+.issuable-sidebar.issuable-affix
+  = form_for [@project.namespace.becomes(Namespace), @project, issuable], remote: true, html: {class: 'issuable-context-form inline-update js-issuable-update'} do |f|
+    .block
+      .title
+        Cross-project reference
+      .cross-project-reference
+        %span#cross-project-reference.has_tooltip{title: 'Cross-project reference'}
+          = cross_project_reference(@project, issuable)
+        = clipboard_button(clipboard_target: 'span#cross-project-reference')
+
+    .block
+      .title
+        %label
+          Assignee:
+        - if can?(current_user, :"admin_#{issuable.to_ability_name}", @project)
+          .pull-right
+            = link_to 'Edit', '#', class: 'edit-link'
+      .value
+        - if issuable.assignee
+          %strong= link_to_member(@project, issuable.assignee, size: 24)
+        - else
+          none
+
+      .selectbox
+        = users_select_tag("#{issuable.class.table_name.singularize}[assignee_id]", placeholder: 'Select assignee', class: 'custom-form-control js-select2 js-assignee', selected: issuable.assignee_id, project: @target_project, null_user: true, current_user: true, first_user: true)
+
+    .block
+      .title
+        %label
+          Milestone:
+        - if can?(current_user, :"admin_#{issuable.to_ability_name}", @project)
+          .pull-right
+            = link_to 'Edit', '#', class: 'edit-link'
+      .value
+        - if issuable.milestone
+          %span.back-to-milestone
+            = link_to namespace_project_milestone_path(@project.namespace, @project, issuable.milestone) do
+              %strong
+                = icon('clock-o')
+                = issuable.milestone.title
+        - else
+          none
+      .selectbox
+        = f.select(:milestone_id, milestone_options(issuable), { include_blank: true }, { class: 'select2 select2-compact js-select2 js-milestone', data: { placeholder: 'Select milestone' }})
+        = hidden_field_tag :issuable_context
+        = f.submit class: 'btn hide'
+
+    - if issuable.labels.any?
+      .block
+        .title
+          %label Labels
+          - if can?(current_user, :"admin_#{issuable.to_ability_name}", @project)
+            .pull-right
+              = link_to 'Edit', '#', class: 'edit-link'
+        .value.issuable-show-labels
+          - issuable.labels.each do |label|
+            = link_to_label(label)
+        .selectbox
+          = f.collection_select :label_ids, issuable.project.labels.all, :id, :name,
+            { selected: issuable.label_ids }, multiple: true, class: 'select2 js-select2', data: { placeholder: "Select labels" }
+
+    = render "shared/issuable/participants", participants: issuable.participants(current_user)
+
+    - if current_user
+      - subscribed = issuable.subscribed?(current_user)
+      .block.light
+        .title
+          %label.light Notifications
+        - subscribtion_status = subscribed ? 'subscribed' : 'unsubscribed'
+        %button.btn.btn-block.btn-gray.subscribe-button{:type => 'button'}
+          %span= subscribed ? 'Unsubscribe' : 'Subscribe'
+        .subscription-status{data: {status: subscribtion_status}}
+          .unsubscribed{class: ( 'hidden' if subscribed )}
+            You're not receiving notifications from this thread.
+          .subscribed{class: ( 'hidden' unless subscribed )}
+            You're receiving notifications because you're subscribed to this thread.
+
+  :javascript
+    new Subscription("#{toggle_subscription_path(issuable)}");
+    new IssuableContext();