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.

config/locales/server.en.yml

@@ -1566,7 +1566,8 @@ en:
     summary_max_results: "Maximum posts returned by 'Summarize This Topic'"
     summary_timeline_button: "Show a 'Summarize' button in the timeline"
 
-    enable_personal_messages: "Allow trust level 1 (configurable via min trust to send messages) users to create messages and reply to messages. Note that staff can always send messages no matter what."
+    enable_personal_messages: "DEPRECATED, use the 'personal message enabled groups' setting instead. Allow trust level 1 (configurable via min trust to send messages) users to create messages and reply to messages. Note that staff can always send messages no matter what."
+    personal_message_enabled_groups: "Allow users within these groups to create messages and reply to messages. Trust level groups include all trust levels above that number, for example choosing trust_level_1 also allows trust_level_2, 3, 4 users to send PMs. Note that staff can always send messages no matter what."
     enable_system_message_replies: "Allows users to reply to system messages, even if personal messages are disabled"
     enable_chunked_encoding: "Enable chunked encoding responses by the server. This feature works on most setups however some proxies may buffer, causing responses to be delayed"
     long_polling_base_url: "Base URL used for long polling (when a CDN is serving dynamic content, be sure to set this to origin pull) eg: http://origin.site.com"
@@ -1889,7 +1890,7 @@ en:
 
     min_trust_to_allow_self_wiki: "The minimum trust level required to make user's own post wiki."
 
-    min_trust_to_send_messages: "The minimum trust level required to create new personal messages."
+    min_trust_to_send_messages: "DEPRECATED, use the 'personal message enabled groups' setting instead. The minimum trust level required to create new personal messages."
     min_trust_to_send_email_messages: "The minimum trust level required to send personal messages via email."
     min_trust_to_flag_posts: "The minimum trust level required to flag posts"
     min_trust_to_post_links: "The minimum trust level required to include links in posts"
@@ -2388,6 +2389,7 @@ en:
       reply_by_email_address_is_empty: "You must set a 'reply by email address' before enabling reply by email."
       email_polling_disabled: "You must enable either manual or POP3 polling before enabling reply by email."
       user_locale_not_enabled: "You must first enable 'allow user locale' before enabling this setting."
+      personal_message_enabled_groups_invalid: "You must specify at least one group for this setting. If you do not want anyone except staff to send PMs, choose the staff group."
       invalid_regex: "Regex is invalid or not allowed."
       invalid_regex_with_message: "The regex '%{regex}' has an error: %{message}"
       email_editable_enabled: "You must disable 'email editable' before enabling this setting."