Indexing new field -- wiki-scoped title tag data

This adds the user-configurable "pagetitle" message to the cross-wiki document.

After testing on dev-indexer-s4, the impact on the cross-wiki core's index appears to be negligible (i.e. within .1 GB difference).

This update will not have an effect on search relevance unless the cross-wiki query service is updated.

We want to store this value in Solr for some experiments the NLP group is working on, since it's currently serving as a canonical source of denormalized data about the wiki. We also think that this could be useful for improved search relevance in the future.