Fix #2971: add reminder events to the bug history
Removed trailing ':' from reminder_sent_to language string

This is for consistency, as except for error messages punctuation is
generally added by the code.

Issue #2971
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 #15470
Optimize code to add reminder recipients to monitoring list
Fix return value of email_bug_reminder()

The function now returns an array containing only the users to whom the
reminder e-mail was actually sent (i.e. excludes failures, blank e-mails,
etc).

In addition, the return value is now an array of user id's instead of
user names, which indeed makes more sense from an API perspective, as
mentioned by Julian Fitzell in 5ea3d0bc837ef14fd66b63d63e5481ea3addcf29,
since the function's only caller is bug_reminder.php which exclusively
relies on user id's.

Fixes #15472
bug_reminder.php does not handle unsent reminders

Reminders are not logged in history (or the bugnote) if the e-mail was not
actually sent to the recipient.

The function now uses email_bug_reminder()'s return value instead of the
full user-provided list of recipients for inclusion in the reminder's
bugnote and to add history entries.

Fixes #15471