Improve handling of reminders' recipients list truncation

Replaced the previous method of truncating the list to a hardcoded
number of entries (50), to a more robust approach based on the size of
the underlying database field (250 chars, note_attr in bugnote table).

Added a message on the REMINDER bugnote, to inform user that the list of
recipients stored with the note, was actually truncated (Users should
refer to the issue's history to see who actually received the reminder).

This functionality relies on a hack, i.e. to indicate that the list was
truncated, bug_reminder.php is not storing the trailing delimiter in the
note_attr field, and this is picked up in bugnote_view_inc.php to
display the note to the user's attention.

Fixes 0015470