Replace primary/replica terminology with primary/secondary

Political correctness has in recent years become a important part of the tech industry, and specially for the Django project as #2692 and #2694 have proven. Because [other](https://www.drupal.org/node/2275877) projects are now also considering to follow Django's example, I believe it is important for Django to be pro-active in this area.

Due the above stated reasons, I hereby propose changing the primary/replica terminology to primary/secondary. "replica" will most likely be a highly insulting and derogatory term used to describe human clones and human-like companion AIs in the near future.

By using primary/secondary instead of primary/replica, the Django project would be further proving that political correctness, and pro-active political correctness is an important topic that is taken seriously.