[FIXED JENKINS-27067] Larger minimum popup menu height

Fix concurrency issues in ConsistentHash

Reflecting more code review comments from
https://github.com/jenkinsci/jenkins/pull/1679

- Jesse wants to keep the variable pointing to 1.6
- Update executable-war so that it fails gracefully if run with Java6 or
  earlier.
- CI build fails with VerifyError due to a regression in JDK.
  To support wide range of development environment, just cut off the
  verifier.
- Have enforcer require 1.7 although javac fails in a kinda obvious way.

Given the minefield that is databinding, that seems prudent.

Starting in 4f24a027 (1.587-SNAPSHOT, before #1443 or stapler #39), nestedDescribableOverridingId passes.
nestedDescribableSharingClass fails given {stapler-class: D3, kind: d3a}.
(Stapler has no way of interpreting kind since it knows nothing of Descriptor, only implementation class names.)
In master (3080573a), both fail in the way reported (only kind is passed in).
After adding stapler-class back, nestedDescribableOverridingId passes again.

newInstancesFromHeteroList should recover gracefully from the case that both kind and $class are missing.

Define additional ConfidentialStore around RSA key pair

Add CONTRIBUTING.md file.

As the first step of Java7 migration, ship Java7 class files without
requiring Java7 runtime.

[FIXED JENKINS-28111] JNLP slave JVM Options are inconsistently applied

[FIXED JENKINS-28115] Division by zero in Executor.getProgress()

```
WARNING: Caught exception evaluating: executor.progress in /ajaxExecutors. Reason: java.lang.reflect.InvocationTargetException
java.lang.reflect.InvocationTargetException
...
	at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.ArithmeticException: / by zero
	at hudson.model.Executor.getProgress(Executor.java:649)
	... 154 more
```

https://github.com/blog/1184-contributing-guidelines

Now that packaging is in https://github.com/jenkinsci/packaging and give...

[JENKINS-28062] Channel listener onClose propagated exceptions

[JENKINS-28058] Don't send a reference to Computer.class over remoting channels

Match the overriden method's actual varargs signature.