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.

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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

GitHub Issue #: 
4621