[admin] Enable the LDAP commands to be performed through the UI

There are three things you can do with LDAP, which are all currently only accessible via the command-line (cf $HUE_ROOT/apps/useradmin/src/useradmin/management/commands):

1) Import user: this just takes a username as input and pulls info from LDAP

2) Import group: this can optionally import all users within the group, so you need a name, as well as a checkbox or something like that

3) Sync: this is just a one-shot button that syncs the imported users and groups with the current state of the LDAP server.

The goal is to make these operation accessible in the UI.