Fix user mention menu on production data

Apparently some of our users in production have no slug, this will
assign an (hopefully unique) random slug to them.

deployment/legacy-migration/maintenance-worker/migration/neo4j/users/users.cql

@@ -94,12 +94,11 @@
       }
     }
 */
-
 CALL apoc.load.json("file:${IMPORT_CHUNK_PATH_CQL_FILE}") YIELD value as user
 MERGE(u:User {id: user._id["$oid"]})
 ON CREATE SET
 u.name       = user.name,
-u.slug       = user.slug,
+u.slug       = COALESCE(user.slug, apoc.text.random(20, "[A-Za-z]")),
 u.email      = user.email,
 u.encryptedPassword   = user.password,
 u.avatar     = replace(user.avatar, 'https://api-alpha.human-connection.org', ''),