feat: add admin screens, permissions and navigation

Seven admin controllers in a "Studia BM" navigation group, guarded by
jz.bm.access_courses, jz.bm.access_schedule and jz.bm.access_materials,
with Polish and English labels. Date and time fields are checked by the
models before a save and answered as 422 on the field.

models/materials/columns.yaml

@@ -0,0 +1,15 @@
+columns:
+    title:
+        label: jz.bm::lang.common.title
+        searchable: true
+    type:
+        label: jz.bm::lang.materials.type
+    course:
+        label: jz.bm::lang.common.course
+        relation: course
+        select: name
+    published_at:
+        label: jz.bm::lang.materials.published_at
+    created_at:
+        label: jz.bm::lang.common.created_at
+        type: datetime

models/materials/fields.yaml

@@ -0,0 +1,46 @@
+# Attached files (system_files, field "files") have no form widget yet; see
+# the TODO on materialsAdmin.AdminFieldRelations.
+fields:
+    title:
+        label: jz.bm::lang.common.title
+        type: text
+        span: left
+        required: true
+    type:
+        label: jz.bm::lang.materials.type
+        type: dropdown
+        span: right
+        required: true
+        options:
+            pdf: jz.bm::lang.material_types.pdf
+            notatka: jz.bm::lang.material_types.notatka
+            link: jz.bm::lang.material_types.link
+            inne: jz.bm::lang.material_types.inne
+    course:
+        label: jz.bm::lang.common.course
+        type: relation
+        nameFrom: name
+        span: left
+        emptyOption: jz.bm::lang.common.none
+    published_at:
+        label: jz.bm::lang.materials.published_at
+        type: text
+        span: right
+        comment: jz.bm::lang.materials.published_at_comment
+    source:
+        label: jz.bm::lang.common.source
+        type: text
+        span: left
+    url:
+        label: jz.bm::lang.common.url
+        type: text
+        span: right
+    tags:
+        label: jz.bm::lang.materials.tags
+        type: relation
+        nameFrom: name
+    body:
+        label: jz.bm::lang.materials.body
+        type: textarea
+        size: giant
+        comment: jz.bm::lang.materials.body_comment