[JENKINS-21532] Fixed wrong placed <br>s.

<br><br> before Extra Configuration block is placed in bad place in HTML and confuses browsers.
Especially combined with [Flexible Publish Plugin] (https://wiki.jenkins-ci.org/display/JENKINS/Flexible+Publish+Plugin), it cause drop-down lists not work.

This patch generates correct HTMLs.

See
https://issues.jenkins-ci.org/browse/JENKINS-21532
https://issues.jenkins-ci.org/browse/JENKINS-21497
for more details.