Removing EntityContent::save

This moves the remaining functionality of EntityContent::save
into WikiPageEntityStore, adressing the bulk of what bug 62493
calls for. Additional cleanup will be done as a followup. In
particuler, EntityContentFactory still contains a number of
methods that shoud really be moved.

Bug: 62493
Change-Id: Ia99a5f29291856c08edf84c7f0eb23fed873ea3e
Merge "Removing EntityContent::save"