Added slack webhook notification functionality

[sheldor1510]

• Created new library UnityWebhook to send notifications to admins on slack channel
• Gets the template from the mail directory, converts into markdown format required by slack
• Delivered message looks like:
  
• Add slack webhook for admin notifications #37

[hakasapl]

This looks good as is, but it would be good to use this library instead: https://github.com/Elephant418/Markdownify

The package will need to be added to the composer.json file. This way, it will support any future HTML we decide to convert to markdown rather than having to homebrew the conversion for every case.

[sheldor1510]

@hakasapl I tried this package along with https://github.com/thephpleague/html-to-markdown as well but it turns out Slack has a different way of formatting markdown content with for bold, links, and other tags. So any package will always be giving the incorrect text back, so I decided to make the function myself and I think it will be easy to update it if further changes are made for the templates.

[hakasapl]

Okay, yes, I see what you mean now. This looks good then, I'll go ahead and merge. Thank you!