Refactor webhost usage

[synercoder]

• Add bug fixed which resulted in email having no body,
• Removed dependency on WebHostBuilder
• Fixed a typo in AddMailDispatcher
• Removed the need to load all assemblies when trying to find a model/identifier combination
• Rearranged DI order + AddSingleton => TryAddSingleton
• Reused usage of data.MailMessage & renamed GenerateMessage to _fillMailMessage & made private
• Made MailSender awaitable
• Moved MailDispatcherOptions to options folder
• Moved IModelIdentifierLister to Abstractions folder
• Refactored the sending of the email to an interface, so DI can be used while sending