sources/ldap: add forward deletion option (#14718)

* sources/ldap: add forward deletion option

* remove unnecessary `blank=True`

* clarify `validated_by` `help_text`

* add indices to `validated_by`

* factor out `get_identifier` everywhere and `get_attributes`

I don't know what that additional `in` check is for, but I'm not about
to find out.

* add tests for known good user and group

* fixup! add tests for known good user and group

* fixup! add tests for known good user and group

web/src/admin/sources/ldap/LDAPSourceForm.ts

@@ -148,6 +148,26 @@ export class LDAPSourceForm extends BaseSourceForm<LDAPSource> {
                     <span class="pf-c-switch__label">${msg("Sync groups")}</span>
                 </label>
             </ak-form-element-horizontal>
+            <ak-form-element-horizontal name="deleteNotFoundObjects">
+                <label class="pf-c-switch">
+                    <input
+                        class="pf-c-switch__input"
+                        type="checkbox"
+                        ?checked=${this.instance?.deleteNotFoundObjects ?? false}
+                    />
+                    <span class="pf-c-switch__toggle">
+                        <span class="pf-c-switch__toggle-icon">
+                            <i class="fas fa-check" aria-hidden="true"></i>
+                        </span>
+                    </span>
+                    <span class="pf-c-switch__label">${msg("Delete Not Found Objects")}</span>
+                </label>
+                <p class="pf-c-form__helper-text">
+                    ${msg(
+                        "Delete authentik users and groups which were previously supplied by this source, but are now missing from it.",
+                    )}
+                </p>
+            </ak-form-element-horizontal>
             <ak-form-group .expanded=${true}>
                 <span slot="header"> ${msg("Connection settings")} </span>
                 <div slot="body" class="pf-c-form">