Custom branded notifications

[nbiton]

This PR includes both the changes for email notifications design in general, and the option for custom branded email notifications.

This is how an email notification looks with default branding:

And this is how an email notification looks with custom hmlet branding:

To enable custom branding, the user Mongo document of the unit's creator needs to include the following object:

{
  ...
  "customEmailBrandingConfig" : {
    "logoUrl" : String,
    "brandName" : String,
    "signatureHtml" : String,
    "signatureText" : String
  }
}

For example:

"customEmailBrandingConfig" : {
  "logoUrl" : "https://www.hmlet.com/static/images/logos/logo_black.png",
  "brandName" : "hmlet",
  "signatureHtml" : "<p>Cheers,<br />Your friends at hmlet",
  "signatureText" : "Cheers, your friends at hmlet"
}

Resolves #814 Resolves #815