minor: Adding backlink support

Resolve "backlink management"

Closes #196

See merge request djangoldp-packages/djangoldp!125

(cherry picked from commit f683b159)

51caf1b2 inbox endpoint on LDPSerializers for receiving notifications
e997443e inbox view for nested fields and for resources
7b7c37dd Merge branch 'master' into 196-backlinks-support
2de1c800 permissions minor efficiency improvement
79f80830 fixed bug with inbox permissions
d1e52ddb Circles have intermediate model for CircleMember
79f976e3 Add Activity success on Projects model (no intermediate manager)
137cebad storing activities and responding 201
629d3534 recursive solution supporting nested objects and intermediate relations
eb0b7dd5 Model.get_model_rdf_type
b367e7f0 ActivityPubService and save receiver
73502788 updated comments
c0e286c9 sending backlinks on m2m relationships
0130aed1 minor corrections
cd37fb49 bounds checking in senders
e7cdcb12 Model.get_or_create
2f5c7fdd responds 404 when it can't store model
7753f719 allow_add_backlinks on model prevent superfluous backlinks
cb71edf4 bugfix missing username from backlinked user
032d0ba3 Added sender and receiver support for Remove Activity
7bb3eabc Added sender and receiver support for Delete activity
7363faa4 fixed typo
6c190af8 backlink_created separated from backlink enabled
22ff182b switched to one-inbox solution
60b29471 logger messages for debugging
908702d9 more detail in logs
2ec6cf28 minor bugfix with serializers
46ddbf2a fix from previous commit
e5428c5b add log message in serializers.py
51d6e259 renamed backlink_created to is_backlink
2bb849de LDPNestedSerializer validation debug messages
dfb55830 extending logs in serializer
482c8739 minor bugfix in m2m_changed listener
d14eb6d8 readability improvement
89598fe3 add test for add activity on pre-existing object
91c42507 checking for activities on any save
46b6cc47 experimental
97dbf6d6 added debug message
eb95d91a another hubl-workaround-493 change
91b53d76 resolve_fk_instances uses get_or_create on distant key
92b01e8c str(uuid4) on generated username get_or_create
a8ebc378 auto_author editable
0699c116 Create activity support
d1ccd098 feature: receiver support for Update activity
b28603d5 sends Create/Update activity on save
86fb0ed5 feature: receiving Follow Activities
f6117c91 auto deleting Followers on object deletion
6bde5c84 feature: sender auto-creates Follower
ee5197cd refactored create/update/delete activities
7943165e removing superfluous debug logs
08d8dd0a some federation info in he README
92e4ae8b Merge branch 'master' into 196-backlinks-support
c1b8e15f minor: federation
56a79508 bugfix: handling exception