WIP: Add administrate default format

[goosys]

• refs Refactor Time-related Fields ahead of upcoming improvements #2702
• The only way to change the time format was to modify the Rails default setting. However, changing it would also affect other parts of the application outside the admin panel. Therefore, I added a dedicated format specifically for Administrate.
  • administrate_date_default administrate_datetime_default administrate_time_default
• Currently, the Time format is fixed as "%I:%M%p", but in Japanese, we would prefer the order "%p %I:%M". I believe other languages may also have their own preferred orders. I have made it possible to change this for each language using administrate_time_default .

[goosys]

At this point, there is a backward compatibility issue.

Since administrate_*_default does not exist in everyone's Rails setup, some initial configuration is required to start using this feature. I feel bad about forcing this initial setup on everyone, so I’d like to discuss if there’s a better way.

1. Write the configuration values for each language in all config/locales/administrate.*.yml files.
2. Create config/locales/date.yml and include only the default (en) settings.
3. Use I18n.localize with default: [:"time.formats.default"] to enable fallback. ref.

Option 1 is very tedious. Option 3 would likely work, but it introduces unnecessary processing if the initial configuration is already set up, so I’d prefer to avoid it if possible.
What do you think?