RFR: 1356: Notifer option to only interact with PRs [v2]

[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.

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Corrected comment

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1288/files
  - new: https://git.openjdk.java.net/skara/pull/1288/files/f795c61f..d3c48cca

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1288&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1288&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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