Fix url resolving for uploads in view

 - Read uploads destination folder from environment variable 'WEBKOCHBUCH_UPLOADS_DESTINATION'
 - Save file paths of images relative to the uploads folder
 - Instantiate environment variable object as java bean during startup and access ist using
dependency injection
 - Added method to create host relative url to uploaded file in its own url context to access
files in the views easily

JIRA:
CG-47
Include new file upload url method in views

JIRA:
CG-47
Update README

Add instructions for correct handling of file uploads

JIRA:
CG-47