Do not paginate if PAGE_SIZE=None

Allow blank/null on radio.html choices

Simplify test settings

fixed missing anchor closing tag

when next_url is none, big part of page html will be rendered under the <a href='#'> as it does not have a closing tag.

Fix lookup_url_kwarg handling in viewsets (Fixes #2591).

The ``lookup_url_kwarg`` is intended to set the name of a field in the
URL regexps when using custom ``lookup_field``, but the routers ignore
it altogether.

Fixed problem with imporing rest_framework.views in apps.py

Fix example of explicit format URL (for real).

Do what I meant to do in #2670:

The link is displayed in the tutorial as "http://example.com/api/items/4.json" but the hyperlink itself and what the example code actually works with are (in my opinion, less pretty) URLs like "http://example.com/api/items/4/.json".

Fix for minor documentation typo

Merge in from upstream

Changed incorrect class name in pagination docs

Remove the 'docs for 3.0' banners.

Update documentation over PAGE_SIZE

Fixed missing word in CursorPagination docs

Support serializing unsaved models with related fields.

This replaces all locations of the old setting `DEFAULT_PAGE_SIZE` for the current setting `PAGE_SIZE`

Remove `MergeDict`

- In both ManyRelatedField, provide an empty return when trying to
  access a relation field if the instance in question has no PK (so
  likely hasn't been inserted yet)
- Add relevant tests
- Without these changes, exceptions would be raised when trying to
  serialize the uncreated models as it is impossible to query
  relations without a PK
- Add test to ensure RelatedField does not regress as currently 
  supports being serialized with and unsaved model

Fix customizing field mappings link