Allow selecting an existing date format for log messages

Description of the need

The "Date format for log messages" field at /admin/config/development/logging requires entry of PHP codes, but we already have existing date/time formats at /admin/config/regional/date-time that people might prefer to use instead.

Proposed solution

Allow people to select from either an existing date format, or a custom format that is entered on the dblog settings page. A custom format entered here would be used for nothing else.

Alternatives that have been considered

• Creating an additional date format presented lots of issues, see https://github.com/backdrop/backdrop-issues/issues/4614 for the full story.
• Original issue: Allow selecting an existing date format for log messages