LDAP Sync blocked user edgecases

Allow GitLab admins to block otherwise valid GitLab LDAP users
(https://gitlab.com/gitlab-org/gitlab-ce/issues/3462)

Based on the discussion on the original issue, we are going to differentiate "normal" block operations to the ldap automatic ones in order to make some decisions when its one or the other.

Expected behavior:

- [x] "ldap_blocked" users respond to both `blocked?` and `ldap_blocked?`
- [x] "ldap_blocked" users can't be unblocked by the Admin UI
- [x] "ldap_blocked" users can't be unblocked by the API
- [x] Block operations that are originated from LDAP synchronization will flag user as "ldap_blocked"
- [x] Only "ldap_blocked" users will be automatically unblocked by LDAP synchronization
- [x] When LDAP identity is removed, we should convert `ldap_blocked` into `blocked`
 
Mockup for the Admin UI with both "ldap_blocked" and normal "blocked" users:
![image](/uploads/4f56fc17b73cb2c9e2a154a22e7ad291/image.png)

There will be another MR for the EE version.

See merge request !2242