Enh: Implement message excerpt filters for JIRA messages, others

Reading through a project list that has a lot of JIRA notifications is not very useful, since the displayed excerpt of the body of every message is:

[ https://issues.apache.org/jira/browse/REEF-1999?page=com.blahblahblah...

Adding pluggable filter API for the display of body excerpts that allows admin-configured filters for the excerpts would be very valuable. A JIRA filter would be reasonably easy to detect a mail is from an automated JIRA notice, and could instead display the first text content of the mail, after the various JIRA headers.

The issue names no files or tests. Start by locating where project-list message excerpts are generated and how administrator configuration is loaded, then trace the existing message parsing path. Done means a configurable excerpt-filter mechanism is available and JIRA notices show their first text content instead of the JIRA headers.

Written by the indexing model from the issue text.