Make PromotionsExtensionPoint an optional extension

[jglick]

As per this comment. 2.26 throws big scary exceptions.

@reviewbybees esp. @daniel-beck, @oleg-nenashev

[jglick]

CI build is failing for unclear reason.

[jglick]

Did you start rewriting history when merging PRs @oleg-nenashev? Irritating because

promoted-builds-plugin$ git pull && git branch -d optional
remote: Counting objects: 10, done.        
remote: Compressing objects: 100% (6/6), done.        
remote: Total 10 (delta 1), reused 1 (delta 1), pack-reused 0        
Unpacking objects: 100% (10/10), done.
From github.com:jenkinsci/promoted-builds-plugin
   3342b0c..af0cb71  master     -> origin/master
Updating 3342b0c..af0cb71
Fast-forward
 src/main/java/hudson/plugins/promoted_builds/Promotion.java                                    | 2 +-
 src/main/java/hudson/plugins/promoted_builds/integrations/jobdsl/PromotionsExtensionPoint.java | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)
warning: deleting branch 'optional' that has been merged to
         'refs/remotes/fork/optional', but not yet merged to HEAD.
Deleted branch optional (was 2478e7c).

which means that there is no verification from Git that the full content of the branch was actually merged: for example, if I had made a follow-up commit in the branch but forgot to push it to the PR, or pushed it after your “merge”, Git will give the same warning. So it is necessary to read the hash of the branch as it stood when I locally deleted it, and then manually compare that to the last commit hash displayed in the PR prior to the merge message.