Elasticsearch: Cast __in lookup values to list

This makes sure lazily-evaluated lists (like querysets) are evaluated so they can be converted to JSON
Reduced title boost to 2. Fixes #553
Improvements to type() PageQuerySet method

This commit makes the type PageQuerySet method include all concrete
models that descend from the specified model in the filter.

This makes this method useful for cases where you have many page types
descending from a single abstract model and want to search across all
these page types (which you can't do in Django normally).
Merge branch 'searchchanges/title-boost-fix' of https://github.com/kaedroho/wagtail into kaedroho-searchchanges/title-boost-fix

Conflicts:
	wagtail/wagtailcore/models.py
release note for 554
Merge branch 'kaedroho-searchchanges/title-boost-fix'
add upgrade considerations note re reduction of title boost
failing test for #465
Merge branch 'es-fix' of https://github.com/kaedroho/wagtail into kaedroho-es-fix

Conflicts:
	wagtail/wagtailsearch/backends/elasticsearch.py
changelog entry for #465
Merge branch 'kaedroho-es-fix'
fix undefined variable when raising the 'cannot filter ElasticSearch results on this field' error
fix undefined variable when raising the 'unrecognised lookup' error
python3 fix for ElasticSearch backend error message
add failing test for #557
Merge branch 'pagequeryset-type-method-improvement' of https://github.com/kaedroho/wagtail into kaedroho-pagequeryset-type-method-improvement
document the PageQuerySet.type method
release note for #557
Fix page reordering in Wagtail Admin
Merge pull request #665 from mope/661-fix-page-reordering

Fix page reordering in Wagtail Admin