FIX: Existing shared drafts should be accessible (#11915)

Disabling shared drafts used to leave topics in an inconsistent state
where they were not displayed as shared drafts and thus there was no
way of publishing them. Moreover, they were accessible just to users
who have permissions to create shared drafts.

This commit adds another permission check that is used for most
operations and the old can_create_shared_draft? remains used just when
creating a new shared draft.

lib/guardian/post_guardian.rb

@@ -142,7 +142,7 @@ module PostGuardian
       can_create_post?(post.topic) &&
       post.topic.category_id == SiteSetting.shared_drafts_category.to_i &&
       can_see_category?(post.topic.category) &&
-      can_create_shared_draft?
+      can_see_shared_draft?
     )
 
     if post.wiki && (@user.trust_level >= SiteSetting.min_trust_to_edit_wiki_post.to_i)

lib/guardian/topic_guardian.rb

@@ -22,8 +22,10 @@ module TopicGuardian
   alias :can_moderate_topic? :can_review_topic?
 
   def can_create_shared_draft?
-    return false unless SiteSetting.shared_drafts_enabled?
+    SiteSetting.shared_drafts_enabled? && can_see_shared_draft?
+  end
 
+  def can_see_shared_draft?
     return is_admin? if SiteSetting.shared_drafts_min_trust_level.to_s == 'admin'
     return is_staff? if SiteSetting.shared_drafts_min_trust_level.to_s == 'staff'
 
@@ -39,7 +41,7 @@ module TopicGuardian
   end
 
   def can_publish_topic?(topic, category)
-    can_create_shared_draft? && can_see?(topic) && can_create_topic_on_category?(category)
+    can_see_shared_draft? && can_see?(topic) && can_create_topic_on_category?(category)
   end
 
   # Creating Methods
@@ -102,7 +104,7 @@ module TopicGuardian
       !topic.private_message? &&
       topic.category_id == SiteSetting.shared_drafts_category.to_i &&
       can_see_category?(topic.category) &&
-      can_create_shared_draft? &&
+      can_see_shared_draft? &&
       can_create_post?(topic)
     )
 
@@ -176,7 +178,7 @@ module TopicGuardian
       return authenticated? && topic.all_allowed_users.where(id: @user.id).exists?
     end
 
-    return false if topic.shared_draft && !can_create_shared_draft?
+    return false if topic.shared_draft && !can_see_shared_draft?
 
     category = topic.category
     can_see_category?(category) &&