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.

db/migrate/20220825054405_fill_personal_message_enabled_groups_based_on_deprecated_settings.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+class FillPersonalMessageEnabledGroupsBasedOnDeprecatedSettings < ActiveRecord::Migration[7.0]
+  def up
+    enable_personal_messages_raw = DB.query_single("SELECT value FROM site_settings WHERE name = 'enable_personal_messages'").first
+    enable_personal_messages = enable_personal_messages_raw.blank? || enable_personal_messages_raw == 't'
+
+    min_trust_to_send_messages_raw = DB.query_single("SELECT value FROM site_settings WHERE name = 'min_trust_to_send_messages'").first
+    min_trust_to_send_messages = (min_trust_to_send_messages_raw.blank? ? 1 : min_trust_to_send_messages_raw).to_i
+
+    # default to TL1, Group::AUTO_GROUPS[:trust_level_1] is 11
+    personal_message_enabled_groups = "11"
+
+    if min_trust_to_send_messages != 1
+      # Group::AUTO_GROUPS[:trust_level_N] range from 10-14
+      personal_message_enabled_groups = "1#{min_trust_to_send_messages}"
+    end
+
+    # only allow staff if the setting was previously disabled, Group::AUTO_GROUPS[:staff] is 3
+    if !enable_personal_messages
+      personal_message_enabled_groups = "3"
+    end
+
+    # data_type 20 is group_list
+    DB.exec(
+      "INSERT INTO site_settings(name, value, data_type, created_at, updated_at)
+      VALUES('personal_message_enabled_groups', :setting, '20', NOW(), NOW())",
+      setting: personal_message_enabled_groups
+    )
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end