Using id for serializing, refreshUser with id

Serialize with only the id field, refreshed user is called by id
Remove serialisation from the Group entity
[#1874] Tweaking the entity provider article, including a new explanation of why serialization might be important
Merge pull request #1878 from WouterJ/adding_twig_reference

[WIP] Add Twig Reference
[#1878] Adding index/map for new twig reference, also making some tweaks to it and adding the `app` global variable
Merge branch '2.0' into 2.1

Conflicts:
	cookbook/security/entity_provider.rst
	reference/forms/twig_reference.rst
Merge branch '2.1'
Merge pull request #1894 from arnaud-lb/hostname

[Routing] added hostname pattern
[Routing] Moving routing component into a directory, will add some other articles shortly
Merge branch '2.0' into 2.1
Merge branch '2.1'
Added title for references
[#1894] Refactoring the hostname routing stuff into a component "cookbook" article with inter-links
[#1878] Tweak per @WouterJ in sha: a0483b4ac8fc5732e5a356a2339dcb243f0eac68
Tweak per @WouterJ in sha: 1d2aefa24c0e0bd368fb073dc449929b7b48e393
Merge pull request #1949 from WouterJ/correct_links_twig_ref

Added title for references
Merge pull request #1896 from kosssi/impoving_json_response

add JsonResponse exemple
[HttpFoundation] Adding a note about JSON responses, which I'll update for the JsonResponse in 2.1
Merge branch '2.0' into 2.1

Conflicts:
	components/http_foundation/introduction.rst
[#1896] Refactoring the new JsonResponse docs into the HttpFoundation component