Added schema for storing message-id and issue-id
Completed implementation for identifying an issue from the References field in mail header.
Refer #0016719
Implemented all the changes suggested by @SL-Gundam
Added configuration item to control using this feature (#16719)
Updated changelog
Merge branch 'master' into 0016719
Added feature to relate bugs if Message-ID is already present in
the database.
This also fixes an error in which the UNIQUE constraint in the
msgids table causing an Application Error
Merged branch upstream changes into 0016719
Removed a redundant method definition which was occured when
merging the conflicts
Merge pull request #8 from manilal/0016719

Identify issue id from mail header(Message-ID and References fields)