Allow enabling LDAP paging for LDAP synchronization queries

[wcarson]

• Gitea version (or commit ref): 1.4.0+rc1-17-g93f34fd
• Git version: 2.16.2
• Operating system: Windows Server 2016
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist:

Description

We have a fairly large LDAP with thousands of users that is configured to allow a max of 1000 results at a time. Without using LDAP paging we get an error like the following:

[...gitea/models/user.go:1388 SyncExternalUsers()] [E] LDAP Search failed unexpectedly! (LDAP Result Code 4 "Size Limit Exceeded": )

As far as I can tell (I'm not familiar with Go), search results paging is supported, so this is definitely doable.