API: add endpoint to show by what objects an object is used (#995)

* core: add used_by API to show what objects are affected before deletion

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web/elements: add support for used_by API

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: add authentik_used_by_shadows to shadow other models

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web: implement used_by API

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* *: fix duplicate imports

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: add action field to used_by api

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web: add UI for used_by action

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web: add notice to tenant form

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: fix naming in used_by

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* web: check length for used_by

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: fix used_by for non-pk models

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* *: improve __str__ on models

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* core: add support for many to many in used_by

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

web/src/pages/tenants/TenantForm.ts

@@ -47,6 +47,7 @@ export class TenantForm extends ModelForm<Tenant, string> {
                 ?required=${true}
                 name="domain">
                 <input type="text" value="${first(this.instance?.domain, window.location.host)}" class="pf-c-form-control" required>
+                <p class="pf-c-form__helper-text">${t`Matching is done based on domain suffix, so if you enter domain.tld, foo.domain.tld will still match.`}</p>
             </ak-form-element-horizontal>
             <ak-form-element-horizontal name="default">
                 <div class="pf-c-check">