Allow anonymous user colors in DB schema

Every authorship now has an author that saves the color.
The author links to the session and optionally the user.
For anonymous users, the session now can be used to reidentify the user.
 If the user is logged-in, the userId is saved into the author table.
 This allows to identify the user even after the session expired.

 A note created by an anonymous user does not have an owner.
 The cardinality was updated to reflect that.

Signed-off-by: David Mehren <git@herrmehren.de>

docs/content/dev/db-schema.plantuml

@@ -74,7 +74,7 @@ entity "revision" {
 entity "authorship" {
   *id : uuid <<generated>>
   --
-  *userId : uuid <FK user>>
+  *authorId : uuid <FK user>>
   *startPos : number
   *endPos : number
   *createdAt : date
@@ -86,11 +86,12 @@ entity "revision_authorship" {
   *authorshipId : uuid <<FK authorship>>
 }
 
-entity "author_colors" {
-  *noteId : uuid <<FK note>>
-  *userId : uuid <<FK user>>
+entity "author" {
+  *id : number <<generated>>
   --
   *color : text
+  sessionID : text <<FK session>>
+  userId : uuid <<FK user>>
 }
 
 
@@ -148,22 +149,23 @@ entity "history_entry" {
   *updatedAt: date
 }
 
-user "1" -- "0..*" note: owner
+user "0..1" -- "0..*" note: owner
 user "1" -u- "1..*" identity
 user "1" -l- "1..*" auth_token: authTokens
 user "1" -r- "1..*" session
 user "1" -- "0..*" media_upload
-user "1" - "0..*" history_entry
+user "1" -- "0..*" history_entry
 user "0..*" -- "0..*" note
-user "1" -- "0..*" authorship
+user "0..1" -- "0..*" author
 
-(user, note) . author_colors
+author "1" -- "0..*" authorship
+author "1" -u- "0..*" session
 
 revision "0..*" -- "0..*" authorship
 (revision, authorship) .. revision_authorship
 
 media_upload "0..*"  -- "1" note
-note "1" - "1..*" revision
+note "1" -d- "1..*" revision
 note "1" - "0..*" history_entry
 note "0..*" -l- "0..*" tag
 note "0..*" -- "0..*" group