Adopt SummerCMS v0.1.1 admin features

Datepickers on native time.Time / lagoon.Date fields, file uploads on
materials, readings and class sessions managed from the course form
(course_id made nullable).

models/materials/fields.yaml

@@ -1,5 +1,3 @@
-# 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
@@ -24,7 +22,10 @@ fields:
         emptyOption: jz.bm::lang.common.none
     published_at:
         label: jz.bm::lang.materials.published_at
-        type: text
+        type: datepicker
+        mode: datetime
+        format: d.m.Y H:i
+        firstDay: 1
         span: right
         comment: jz.bm::lang.materials.published_at_comment
     source:
@@ -39,6 +40,13 @@ fields:
         label: jz.bm::lang.materials.tags
         type: relation
         nameFrom: name
+    files:
+        label: jz.bm::lang.materials.files
+        type: fileupload
+        mode: file
+        maxFilesize: 50
+        maxFiles: 20
+        useCaption: true
     body:
         label: jz.bm::lang.materials.body
         type: textarea