[JENKINS-7494] Adding UserAvatarResolver to make it possible for plugins to add avatar images to user pages

[JENKINS-5335] Add html doctype on Jenkins to avoid quirks mode.

Stop a matrix-project build

Added more diagnositics about how ping terminated a connection.

cloud.

- calling prebuild after build wrappers to align the semantics with freestyle projects.

build.xml that can't be deserialized

Handle resolves-to="com.google.common.collect.ImmutableList$SerializedForm"
manually and construct a suitable list. Any subsequent serialization of the
class will use the direct form of serialization.

This is a partial fix for JENKINS-10903. Any new builds will
be serialized correctly.

Based on ImmutableMapConverter.java