Add a link to the channel in the channel publish email

[rtibbles]

Currently, the channel publish email just references the channel name, id, token, and version notes: https://github.com/learningequality/studio/blob/unstable/contentcuration/contentcuration/templates/registration/channel_published_email.txt#L6

This could be slightly improved by adding the full text of the hyperlink into the email as well, allowing users to either copy paste the link, or to open it in email clients that support links.

We should avoid adding HTML to the email for the widest support of different email clients, and to prevent it being scrubbed by email sanitization where possible.