owncloud/core

Change: Allow to temporarily ignore invalid federated shares

This change is targeted mostly at tightly federated setups

Currently, if federated share is invalid or api endpoint returns not found, 
availability check would validate whether this is a problem with a server and 
if checks complete that given share is removed. 
However, in some cases these checks might not be enough (e.g. complex 
migrations in tightly federated setups), and in that case invalidation 
behaviour can be disabled using below app setting:

files_sharing.enable_cleanup_invalid_external_shares='no'

https://github.com/owncloud/core/pull/40503
https://github.com/owncloud/enterprise/issues/5427