Add number of edits by user per namespace

[alex-mashin]

Currently, there is ___USEREDITCNT property that gives access to the total number of edits by a user.

It would be useful to allow to get number of edits by a user in a certain namespace.

This can be done by adding a property of the type Record or a subobject, with two fields: namespace number and number of edits. There will be one such record or subobject per user per namespace.

Setting the property will require querying page, revision_actor_temp and actor tables. Updating the property could be done by a hook RevisionRecordInserted.