* Send notifications to partecipants in issue comments
Closes#1216
Includes test (still failing)
* Do not include "labelers" to participants
Fix test to expect what GetParticipants return
a transaction session was in progress. MailParticipants was failing
silently because of the SQLITE_LOCKED error. Make sure failures in
MailParticipants enter the log, and pass on the transaction context.
issue: let caller pass in database context, and use it
issue_comment: obtain database context to pass to UpdateIssueMentions
issue_comment: log any error from call to MailParticipants
issue_mail: pass on database context to UpdateIssueMentions
