feat: Organisations admin screen with the avatar and the members manager

- controller golem15.user.organisations behind golem15.users.access_users
- list and form (name, slug preset from the name, description, avatar)
- members relation manager: link sets and unlink clears users.organisation_id
- deleting an organisation releases its members first
- Organisation gains Fillable, Rules, MorphName, AttachRelations, BeforeValidate
  and the Members relation; models.Slugify
- navigation side item organisations
- TestAdminOrganisations, TestAdminOrganisationMembers, TestSlugify

models/organisation/columns.yaml

@@ -0,0 +1,14 @@
+columns:
+    id:
+        label: golem15.user::lang.organisation.id
+        invisible: true
+    name:
+        label: golem15.user::lang.organisation.name
+        searchable: true
+    slug:
+        label: golem15.user::lang.organisation.slug
+        searchable: true
+    created_at:
+        label: backend::lang.list.column_created
+        type: datetime
+        sortable: true

models/organisation/fields.yaml

@@ -0,0 +1,30 @@
+fields:
+    name:
+        label: golem15.user::lang.organisation.name
+        type: text
+        span: left
+        required: true
+    slug:
+        label: golem15.user::lang.organisation.slug
+        comment: golem15.user::lang.organisation.slug_comment
+        type: text
+        span: right
+        preset:
+            field: name
+            type: slug
+    description:
+        label: golem15.user::lang.organisation.description
+        type: textarea
+        size: small
+    avatar:
+        label: golem15.user::lang.organisation.avatar
+        type: fileupload
+        mode: image
+        imageWidth: 120
+        imageHeight: 120
+    members:
+        type: relation-manager
+        relation: members
+        tab: golem15.user::lang.organisation.members
+        span: full
+        context: update