Add check to newsletters to prevent /admin/newsletters urls
Closes #ISSUE
Previous behaviour
Steps to reproduce:
- Add text to the newsletter with a url lacking
http(s)
- No problems, but the url in the newsletter is
base_url/newsletters/newsletter/(id)/(url)
New behaviour
Steps to validate that it works:
- Add text to the newsletter with a url lacking
http(s)
- Get an error preventing this from happening