### What problem does this PR solve?
As issue #3268 mentioned, "Chun not found!" exception will occur,
especially during the teamwork of knowledge bases.
### The reason of this bug
"tenants" are the people on current_user's team, including the team
owner itself. The old one only checks the first "tenant", tenants[0],
which will cause error when anyone editing the chunk that is not in
tenants[0]'s knowledge base.
My modification won't introduce new errors while iterate all the tenant
then retrieve knowledge bases of each.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
11e6d84d46