[server] allow create tmp files through api

Seafile server doesn't allow create new files with names
like 'doc.tmp'. But when a user mounts his seafile account
through webdav, some program would not work properly because it
may try to create tmp files.

For this reason, we remove the aformentioned restriction. However,
seafile desktop client would still ignore those tmp files when syncing.
Merge pull request #886 from haiwen/allow-create-tmp-files-on-server

[server] allow create tmp files through api