[#632] Return full path to images

The Base64ImageField only returns the internal path to the uploaded file
location. Fix by prepending settings.MEDIA_URL.
[#632] Add OrganisationExtraSerializer

OrganisationExtraSerializer is used by ProjectUpdateExtraSerializer
(indirectly, its actual use is in UserExtraSerializer).
[#632] Add UserExtraSerializer

UserExtraSerializer is used by ProjectUpdateExtraSerializer
[#632] Add ProjectUpdateExtraSerializer

ProjectUpdateExtraSerializer returns data not only about an Update, but
also about the User that created the update and the Organisation of that
User.

This is intended to replace the Tastypie resource
/api/v1/project_update_extra/.
[#632] Add ProjectUpdateExtraSerializer to rest/serializers

Note that the other "Extra" serializers are not added as they are only
for internal use by ProjectUpdateExtraSerializer.
[#632] Add OrganisationLocationSerializer

OrganisationLocationSerializer is used by OrganisationExtraSerializer.
[#632] Add comment to ProjectUpdateExtraSerializer
[#632] Add ProjectUpdateLocationExtraSerializer

ProjectUpdateLocationExtraSerializer is used by
ProjectUpdateExtraSerializer.
[#632] Remove empty Meta.fields()
[#632] Extend BaseRSRSerializer.__init__()

BaseRSRSerializer now adds an "absolute_url" field if the associated
model defines the get_absolute_url() method.
[#632] Add ProjectUpdateExtraViewSet
[#632] Add ProjectUpdateExtraViewSet to rest/views
[#632] Add new endpoints to urls.py

Add ProjectUpdateExtraViewSet and ProjectUpdateLocationViewSet to
urls.py as project_update_extra and project_update_location respectively.