Rework the replacement settings

[theofidry]

• Add documentation and tests for the replacement map
• Deprecate the setting datetime_format in favour of the new setting datetime-format
• [BC break] The datetime value used for the datetime placeholder is now always in UTC
• [BC break] The settings datetime-format and datetime_format are now always evaluated even if the datetime setting is not used or null
• [BC break] The settings datetime-format and datetime_format now throw an exception when the format is invalid