Add check to newsletters to prevent /admin/newsletters urls

Ghost User
2018-11-02T15:42:49+01:00

Closes #ISSUE

Previous behaviour

Steps to reproduce:

1. Add text to the newsletter with a url lacking http(s)

2. No problems, but the url in the newsletter is base_url/newsletters/newsletter/(id)/(url)

New behaviour

Steps to validate that it works:

1. Add text to the newsletter with a url lacking http(s)

2. Get an error preventing this from happening