Allow usage of WYSIWYG in maintenance message

Problem/Motivation

When creating a custom message for the maintenance page, you can use HTML at /admin/config/development/maintenance. The purpose of this issue is to enable the WYSIWYG editor on this page.

User interface changes

Before:

Expected after:

Impacts

1. Update the system.schema and the corresponding system.maintenace configuration.
2. Add hook_update_N() for this.
3. Update MaintenanceForm to allow WYSIWYG.
4. Update the message display in Maintenance page to let the message value being displayed.
5. Update tests
6. Update translation tests
7. Update update process (from D6/D7 to D8)
8. Update the update process tests.

Data model changes

Before message is now a text_format instead of text in system.schema.yml, the system.maintenance.yml is impacted. message key is now an array of value / format.

Before:

message: '@site is currently under maintenance. We should be back shortly. Thank you for your patience.'
langcode: en

After:

message:
  value: '@site is currently under maintenance. We should be back shortly. Thank you for your patience.'
  format: plain_text
langcode: en