RFR: Refactoring in preparation for SKARA-1146

[Erik Joelsson]

While working on SKARA-1146, I felt that I needed to do some refactoring. To ease the review process, I've separated those changes into a separate PR. Nothing should change functionally from this change. Here is what I've done:

* Created String constants for the special custom fields we have in JBS.
* In IssueNotifier, created a method for finding the fixVersion for a branch. This code block is actually copied and sprinkled all over the code base, but for now, I will just unify the copies in the same class.

-------------

Commit messages:
 - Refactor out common functionality
 - SKARA-1146

Changes: https://git.openjdk.java.net/skara/pull/1270/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1270&range=00
  Stats: 110 lines in 10 files changed: 38 ins; 26 del; 46 mod
  Patch: https://git.openjdk.java.net/skara/pull/1270.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1270/head:pull/1270

PR: https://git.openjdk.java.net/skara/pull/1270