Account for whitespace in the middle of links when checking javadoc links

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1627410 13f79535-47bb-0310-9956-ffa450edef68
Update versions in trunk's site, update migration guide

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1627418 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5971: Create addBackcompatIndexes.py script to build and add backcompat test indexes

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1627419 13f79535-47bb-0310-9956-ffa450edef68