[IMP] base_setup, mail, hr: allow to configure company alias domains

PURPOSE

Allow alias domains to be multiple, notably to be used in a multi company
environment where each company has its own alias domain.

SPECIFICATIONS

Update settings to configure your company's current alias domain instead
of the global 'alias.catchall.domain' configuration parameter. As domain
configuration is globally done per company, this is now a related on the
company. Advanced configuration and management can still be done manually
in settings.

Field 'external_email_server_default' triggering display of mail configuration
is moved to mail. It is defined in 'base_setup' but used only in mail hence
moving the field to be coherent.

Task-36879 (Mail: Support Multi Domains Aliases)

Part-of: odoo/odoo#76734

addons/base_setup/models/res_config_settings.py

@@ -15,9 +15,6 @@ class ResConfigSettings(models.TransientModel):
     user_default_rights = fields.Boolean(
         "Default Access Rights",
         config_parameter='base_setup.default_user_rights')
-    external_email_server_default = fields.Boolean(
-        "Use Custom Email Servers",
-        config_parameter='base_setup.default_external_email_server')
     module_base_import = fields.Boolean("Allow users to import data from CSV/XLS/XLSX/ODS files")
     module_google_calendar = fields.Boolean(
         string='Allow the users to synchronize their calendar  with Google Calendar')