* [Mailman2Bridge.php] enable PCRE_MULTILINE pattern modifier
Enable PCRE_MULTILINE pattern modifier on mbox content parsing. Without it parsing monthly archives results in only a single message each.
* [Mailman2Bridge.php] extend mbox "From_ lines" pattern
Extend PCRE pattern matching individual "From_ lines" used to split single messages in mbox content.
In addition to the matching line having to start with 'From ' it now also has to end with time and date (hh:mm:ss yyyy).
This makes the pattern slightly more robust against accidental matches when a line within the actual message body starts with 'From ' which Mailman 2 (Pipermail) may not be configured to disambiguate.
* [Mailman2Bridge.php] remove trailing slash from URI constant
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Co-authored-by: enwu <108224417+8279279374@users.noreply.github.com>
