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/class_sessions/columns.yaml

@@ -0,0 +1,16 @@
+columns:
+    starts_at:
+        label: jz.bm::lang.sessions.starts_at
+    ends_at:
+        label: jz.bm::lang.sessions.ends_at
+    course:
+        label: jz.bm::lang.common.course
+        relation: course
+        select: name
+        searchable: true
+    room:
+        label: jz.bm::lang.sessions.room
+        searchable: true
+    is_online:
+        label: jz.bm::lang.sessions.is_online
+        type: switch

models/class_sessions/fields.yaml

@@ -0,0 +1,41 @@
+fields:
+    course:
+        label: jz.bm::lang.common.course
+        type: relation
+        nameFrom: name
+        span: left
+        required: true
+    meeting:
+        label: jz.bm::lang.sessions.meeting
+        type: relation
+        nameFrom: number
+        span: right
+        emptyOption: jz.bm::lang.common.none
+    starts_at:
+        label: jz.bm::lang.sessions.starts_at
+        type: text
+        span: left
+        required: true
+        comment: jz.bm::lang.common.datetime_comment
+    ends_at:
+        label: jz.bm::lang.sessions.ends_at
+        type: text
+        span: right
+        required: true
+        comment: jz.bm::lang.common.datetime_comment
+    room:
+        label: jz.bm::lang.sessions.room
+        type: text
+        span: left
+    kind:
+        label: jz.bm::lang.sessions.kind
+        type: text
+        span: right
+    is_online:
+        label: jz.bm::lang.sessions.is_online
+        type: switch
+        span: left
+    notes:
+        label: jz.bm::lang.common.notes
+        type: textarea
+        size: small