Add enable incremental sync flag to Active Directory configuration UI

Active Directory supports incremental sync, but it doesn't work in all situations (See http://confluence.atlassian.com/display/CROWD/Configuring+Caching+for+an+LDAP+Directory#ConfiguringCachingforanLDAPDirectory-Notes). We need an "Enable incremental synchronisation" checkbox in the UI, so customers with affected configurations will not lose LDAP modifications. The checkbox should be enabled by default. Some back-end work is also required for the checkbox to have effect.

This issue should be solved at the same time as https://studio.atlassian.com/browse/EMBCWD-833, as that issue also requires some changes in Crowd.