Bump stapler to lastest.

Using https://github.com/stapler/stapler/pull/45 if the request contain jsonp parameter send the correct MIME type to avoid the error:
Refused to execute script from 'JENKINS_URL' because its MIME type ('application/json') is not executable, and strict MIME type checking is enabled.

show « check now » button on PluginManager updates and available pages

(cherry picked from commit cf2b02dc)

Conflicts:
	changelog.html

(cherry picked from commit 87ab95e7)

TransientActionFactory should implement ExtensionPoint

Simplify code a bit using Collections.reverseOrder()

Conflicts:
	test/src/test/java/hudson/model/UpdateCenter2Test.java

My plan is to remove this in a while.

With the server-side download, the test needs to induce the update
center metadata retrieval in a separate manner.

Conflicts:
	pom.xml