add: plaintext email; improved templating

configs/config.template.json

@@ -1,5 +1,7 @@
 {
+  "WebsiteTitle": "My Carsharing Site",
   "BaseURL": "https://domain.de",
+  "Environment": "dev",
   "db": {
     "Path": "data/db.sqlite3"
   },
@@ -8,13 +10,13 @@
     "User": "username",
     "Password": "password",
     "Port": 465,
-    "Mailtype": "html",
     "AdminEmail": "admin@server.com"
   },
   "templates": {
     "MailPath": "templates/email",
     "HTMLPath": "templates/html",
-    "StaticPath": "templates/css"
+    "StaticPath": "templates/css",
+    "LogoURI": "/images/LOGO.png"
   },
   "auth": {
     "APIKey": ""