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.
