LDAP: introduce experienced mode for wizard to enable admins to operate without too many disturbing automatic LDAP requests

Fixes https://github.com/owncloud/core/issues/11448

It introduces a new checkobx on the Server tab, " I am familiar with LDAP?".
When this is checked, following happens:
- all LDAP filter go into Raw Mode (direct entry) (and again in reload).
- automatic counting of users and groups is disabled. It will be triggered by a new button on it's own. This button (on User and Group  tabs) is available in experienced mode only.
- switching from raw mode to assisted mode will ask for confirmation (experienced mode only).

When this is unchecked again, the usual behaviour is restored. 

By default the checkbox is unchecked.

Intensive testing required, esp. when creating new configurations and on switching configurations. Please test @craigpg @jnfrmarks @PVince81 or others.
[trivial] You should not need to delete the filter, unless you expect the `new` operation below to fail and reach an exception block, which would then still have the old value.