chore(release): update cdn version
docs(changelog): release notes for 1.2.0
docs(guide/migration): add migration guide
chore(docs): allow periods in doc shortNames
docs(guide/migration): fix link to #2500
docs(guide/migration): fix up internal links
chore(release): update version number for next round of development
feat(jqLite): implement the `empty` method

jQuery's elem.html('') is way slower than elem.empty(). As clearing
element contents happens quite often in certain scenarios, switching
to using .empty() provides a significant performance boost when using
Angular with jQuery.