Dynamically check `:admin_*` ability in issuable context

Closes #2001

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

@@ -8,7 +8,7 @@
       - else
         none
     .issuable-context-selectbox
-      - if can?(current_user, :admin_issue, @project)
+      - if can?(current_user, :"admin_#{issuable.class.to_s.underscore}", @project)
         = 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)
 
   %div.prepend-top-20.clearfix
@@ -24,7 +24,7 @@
       - else
         none
     .issuable-context-selectbox
-      - if can?(current_user, :admin_issue, @project)
+      - if can?(current_user, :"admin_#{issuable.class.to_s.underscore}", @project)
         = f.select(:milestone_id, milestone_options(issuable), { include_blank: 'Select milestone' }, {class: 'select2 select2-compact js-select2 js-milestone'})
         = hidden_field_tag :issuable_context
         = f.submit class: 'btn hide'