RFR: Refactoring in preparation for SKARA-1146 [v2]

[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.

Erik Joelsson has updated the pull request incrementally with two additional commits since the last revision:

 - Rename variable
 - Remove unwanted change

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

Changes:
  - all: https://git.openjdk.java.net/skara/pull/1270/files
  - new: https://git.openjdk.java.net/skara/pull/1270/files/9ea6deed..bc66d6e7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1270&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1270&range=00-01

  Stats: 4 lines in 3 files changed: 0 ins; 2 del; 2 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