Add CommunitySolution

* Also slightly refactor some JS files

app/models/community_solution.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class CommunitySolution < ApplicationRecord
+  belongs_to :exercise
+  has_many :community_solution_locks
+  has_many :community_solution_contributions
+  has_and_belongs_to_many :users, polymorphic: true, through: :community_solution_contributions
+  has_many :files, class_name: 'CodeOcean::File', through: :community_solution_contributions
+
+  def to_s
+    "Gemeinschaftslösung für #{exercise}"
+  end
+end