#18659 -- Deprecated request.REQUEST and MergeDict

See [ticket 18659](https://code.djangoproject.com/ticket/18659) on removing request.REQUEST and MergeDict. I've moved the codebase away from using either of them. In the progress I've also added deprecation notices when those are being used. Please advice me if this is the correct approach.

The documentation needs to reflect this deprecation, if this PR is up to par, I make those changes.