Follow-up: Introduce DELETE

[der-john]

DELETE has been implemented in #2365.

Some problems discussed in #2336 remain.

• backup 'deleted' resources for 2 weeks
  i suggest extending the audit log and add a script to make it usable by sysadmins (@joka)
• remove 'deleted' flag form metadata sheet (only used in Debate Workbench / Document.ts)
  as far as I can see. I suggest it be replaced by hide ( @der-john)
• check if delete permission is given
• remove 'deleted field in IMetadata sheet in backend
• ([ ] enable 'removed'/ 'removed_child' notification for the websocket api see Websocket is not notified about deleted resources (wrong resource path) #2700 )
• remove special 'hidden' field for metadatda sheet in options requests (not needed anymore)
• Fix delete in pools for versions (e.g. blog entries)

[joka]

I merged the remaining todos from #2365

[xi]

I elevated this to high prio because of the backup feature which is critical

[der-john]

I was wrong about Debate Workbench / Document.ts: there is no delete action relating to the backend there at all (only a frontend attribute called deleted that makes a paragraph not be posted in the next version of the document).

[joka]

Removed high prio cause backup feature is implemented