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