RFR: 1356: Notifer option to only interact with PRs

[Erik Joelsson]

The "pronly" option was introduced in [SKARA-445](https://bugs.openjdk.java.net/browse/SKARA-445) with the intention of support running just a PR based IssueNotifier, which includes resolving issues when a PR is integrated.

We need to run an IssueNotifier without it trying to resolve issues and without reacting to repo events. To support this, I've introduced a new config parameter on IssueNotifier: "resolve". Since we only need this when pronly is true, I didn't bother implementing support for that case and instead added a validation check in the factory so we don't accidentally try to run with an unsupported configuration.

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Commit messages:
 - Adjusted patch
 - SKARA-1356

Changes: https://git.openjdk.java.net/skara/pull/1288/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1288&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1356
  Stats: 28 lines in 3 files changed: 25 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1288.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1288/head:pull/1288

PR: https://git.openjdk.java.net/skara/pull/1288