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Errors after upgrade to DNN 8?

When you get this error:

Could not load file or assembly 'System.Web.Helpers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Make sure the following is present in the web.config

<dependentAssembly>
  <assemblyIdentity name="System.Web.Helpers" publicKeyToken="31bf3856ad364e35" />
  <bindingRedirect oldVersion="1.0.0.0-3.0.0.0" newVersion="3.0.0.0" />
</dependentAssembly>