started creating module ui, workign on assignments

Management/Features/Modules/ModuleManager.cs

@@ -0,0 +1,22 @@
+public class ModuleManager : IModuleManager
+{
+  public IEnumerable<CourseModule> Modules { get; internal set; } = new CourseModule[] { };
+
+  public void AddAssignment(int moduleIndex, LocalAssignment assignment)
+  {
+    var newAssignments = Modules.ElementAt(moduleIndex).Assignments.Append(assignment);
+    var newModule = Modules.ElementAt(moduleIndex) with { Assignments = newAssignments };
+    if (newModule == null)
+      throw new Exception($"cannot get module at index {moduleIndex}");
+
+    Modules = Modules
+      .Take(moduleIndex)
+      .Append(newModule)
+      .Concat(Modules.Skip(moduleIndex + 1));
+  }
+
+  public void AddModule(CourseModule newModule)
+  {
+    Modules = Modules.Append(newModule);
+  }
+}