[JENKINS-16845] Refinement to hotfix: if build field has already been cleared due to earlier warning, do not throw a new NPE.

This bug indicates that people are having trouble understanding who's
responsible for aborting the build. This change records that
information through the logger.

A recent Stapler update caused the Jelly facet to ignore *.xml.
So need to use raw j:include (as plugins were doing), not st:include (as core did).

Most notable, cvs 1.6 → 2.8, to get the critical fix of removing the mail address resolver.

(cherry picked from commit 9858bf1e)

This is deemed important because we ship Jenkins without security setting by default.

For multiple line warning messages, this makes them little bit spaced, but I think it's OK.

Plus more simplifications

Using prototype.js to simplify the code

On Chrome (and possibly others), an inline element like span doesn't enlarge
its size to cover its content, so the 1px place holder wasn't taking effect.

To ensure the minimum height of 16px, make it an inline-block element. I'm
not entirely sure how an inline-block element with no fixed width would behave,
but with a bit of casual testing, the text inside seems to wrap OK.