Incorporate Support For Templated Messages

[mitchelsellers]

This is different than the current implementation which is focused on the higher-level email. This is the actual CONTENT of the email.

Current usage outside of this project is

public interface IEmailMessageService
    {
        Task<string> BuildMessage(string messageTemplateName, List<KeyValuePair<string, string>> replacements);
    }

    public class EmailMessageService : IEmailMessageService
    {
        private readonly IWebHostEnvironment _environment;
        private readonly ITimeProvider _timeProvider;

        public EmailMessageService(IWebHostEnvironment environment, ITimeProvider timeProvider)
        {
            _environment = environment;
            _timeProvider = timeProvider;
        }

        public async Task<string> BuildMessage(string messageTemplateName,
            List<KeyValuePair<string, string>> replacements)
        {
            var templateBuilder = new StringBuilder(await File.ReadAllTextAsync(Path.Combine(_environment.WebRootPath,
                $"emailmessages/{messageTemplateName}.html")));

            foreach (var item in replacements)
                templateBuilder.Replace(item.Key, item.Value);
            return templateBuilder.ToString();
        }

    }