RFR: 1660: Add confname field to slack log config

[Erik Joelsson]

The Slack log handler currently takes a "username" value as input which if set is sent to Slack as the user name of the message. With modern webhooks, it unfortunately seems that we have lost the feature of overriding the user name of a message. Our current configs put a descriptive string identifying the bot configuration in this field, which we can no longer see.

I want to add a specific new configuration field to the Slack log handler for the "configname". If set, this will appear first in every Slack message.

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Commit messages:
 - SKARA-1660

Changes: https://git.openjdk.org/skara/pull/1413/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1413&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1660
  Stats: 41 lines in 3 files changed: 26 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/skara/pull/1413.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1413/head:pull/1413

PR: https://git.openjdk.org/skara/pull/1413