Next item structure works (#5380)

* Use "LEFT JOIN" to always fetch the item. Needed for update routines.

* New conversion routine that now covers every item

* Post update is now activated

* We now use a hash based upon RIPEMD-320 for content and activity

* The hash doesn't contain the plink anymore

* Legacy item fields are now "null"able

* New hash function for a server unique item hash

* Introduction of the legacy mode (usage of old item fields)

* Code simplification

* We don't need the "uri" fields anymore in item-activity and item-content

* Use the "created" and not the "received" date for the hash

* Avoiding several notices

* Some more warnings removed

* Improved uri-hash / Likes on Diaspora are now getting a creation date

* Corrected the post update version

* Ensure an unique uri-hash

* Don't delete orhaned item data at the moment

* Partly reworked, due to strange behaviour

* Some more parts reworked

* Using the uri currently seems to be more reliable

* Using the uri here as well

* Use the hash values again

* Grouped item fields in different categories

* Notices again

* use the gravity (we always should)

* Added hint for disabled post updates

* Notices ...

* Issue #5337: Personal notes are displayed again

* Use the gravity again