Restricted context search

* Add a tags field to the Context object
* Add a tags search parameter
* Docs and Tests
* Better query on multiple tags using $all
Add tag field to related docs
Update default field values

* Set generator field on activity
* Set tags field on context
* Test fields with defaults get created when not provided
* Fix set defaults to accept None as a default value
Add objectType to base objects
Cleanup of fields stored and showed on searches

* Add an option to the MADMaxCollection.search to squash the _private fields when flattening
* Remove unnecessary fields to be included in the build process of activities and comments
* Search users by username not by _id
* Update and fix some tests
Apply squashing of private fields to more searches
Check if the field exists before deleting
Update docs with latest expected responses
Cleanup unused stuff
Unify actor and author fieldnames >> actor
Add creator field to all objects
Add owner field to future edit/delete permission check
Check field exists before deleting
Update maxui docs and ownership descriptions