FEATURE: Introduce personal_message_enabled_groups setting (#18042)

This will replace `enable_personal_messages` and
`min_trust_to_send_messages`, this commit introduces
the setting `personal_message_enabled_groups`
and uses it in all places that `enable_personal_messages`
and `min_trust_to_send_messages` currently apply.

A migration is included to set `personal_message_enabled_groups`
based on the following rules:

* If `enable_personal_messages` was false, then set
  `personal_message_enabled_groups` to `3`, which is
  the staff auto group
* If `min_trust_to_send_messages` is not default (1)
  and the above condition is false, then set the
  `personal_message_enabled_groups` setting to
  the appropriate auto group based on the trust level
* Otherwise just set `personal_message_enabled_groups` to
  11 which is the TL1 auto group

After follow-up PRs to plugins using these old settings, we will be
able to drop the old settings from core, in the meantime I've added
 DEPRECATED notices to their descriptions and added them
to the deprecated site settings list.

This commit also introduces a `_map` shortcut method definition
for all `group_list` site settings, e.g. `SiteSetting.personal_message_enabled_groups`
also has `SiteSetting.personal_message_enabled_groups_map` available,
which automatically splits the setting by `|` and converts it into
an array of integers.

lib/guardian/group_guardian.rb

@@ -33,8 +33,12 @@ module GroupGuardian
   def can_see_group_messages?(group)
     return true if is_admin?
     return true if is_moderator? && group.id == Group::AUTO_GROUPS[:moderators]
+    return false if user.blank?
 
-    SiteSetting.enable_personal_messages? && group.users.include?(user)
+    # TODO (martin) Remove enable_personal_messages here once plugins have been changed.
+    (SiteSetting.enable_personal_messages ||
+      user.in_any_groups?(SiteSetting.personal_message_enabled_groups_map)) &&
+      group.users.include?(user)
   end
 
   def can_associate_groups?

lib/guardian/post_guardian.rb

@@ -50,10 +50,10 @@ module PostGuardian
         (!SiteSetting.allow_flagging_staff?) &&
         post&.user&.staff?
 
+      # TODO (martin) Remove enable_personal_messages here once plugins have been changed.
       if action_key == :notify_user &&
-         (!SiteSetting.enable_personal_messages? ||
-         !@user.has_trust_level?(SiteSetting.min_trust_to_send_messages))
-
+          (!@user.in_any_groups?(SiteSetting.personal_message_enabled_groups_map) ||
+           !SiteSetting.enable_personal_messages)
         return false
       end
 

lib/guardian/tag_guardian.rb

@@ -15,6 +15,7 @@ module TagGuardian
     return false if @user.blank?
     return true if @user == Discourse.system_user
 
+    # TODO (martin) Change to pm_tags_allowed_for_groups_map
     group_ids = SiteSetting.pm_tags_allowed_for_groups.to_s.split("|").map(&:to_i)
     group_ids.include?(Group::AUTO_GROUPS[:everyone]) || @user.group_users.exists?(group_id: group_ids)
   end

lib/guardian/topic_guardian.rb

@@ -183,7 +183,8 @@ module TopicGuardian
   end
 
   def can_convert_topic?(topic)
-    return false unless SiteSetting.enable_personal_messages?
+    # TODO (martin) Remove enable_personal_messages here once plugins have been changed.
+    return false unless SiteSetting.enable_personal_messages? || @user.in_any_groups?(SiteSetting.personal_message_enabled_groups_map)
     return false if topic.blank?
     return false if topic.trashed?
     return false if topic.is_category_topic?