Make ContentContainer.contents thread-safe

[jglick]

I observed an error

java.util.ConcurrentModificationException
	at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1043)
	at java.base/java.util.ArrayList$Itr.next(ArrayList.java:997)
	at com.cloudbees.jenkins.support.SupportPlugin.writeBundle(SupportPlugin.java:374)
	at com.cloudbees.jenkins.support.SupportPlugin.writeBundle(SupportPlugin.java:330)
	at com.cloudbees.jenkins.support.SupportPlugin$PeriodicWorkImpl.lambda$doRun$0(SupportPlugin.java:888)

where the line number points to

support-core-plugin/src/main/java/com/cloudbees/jenkins/support/SupportPlugin.java

Line 395 in ce1fc0f

for (Content content : contents) {

I suppose some component must have called Container.add outside the dynamic scope of Component.addContents (from another thread); unusual but not specifically forbidden. Safer to make the list of contents be thread-safe, and have appendManifestContents produce a snapshot.