Fix #1043: use customfield_10319 as default severity field

[leplatrem]

Fix #1043

From Jira admin:

Note: these have two separate field names.
* customfield_10716 is “Requested Severity Level”
* customfield_10319 is “Severity ” (with the space at the end of Severity
We’ll be working on removing additional trailing spaces and duplicates in our cleanup, but these two custom fields have separate names