Add filter for `shareapi_allow_share_dialog_user_enumeration`

This adjusts the contacts menu to also support searching by email address which is relevant in scenarios where no UID is known such as LDAP, etc.

Furthermore, if `shareapi_allow_share_dialog_user_enumeration` is disabled only results are shown that match the full user ID or email address.

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>