Add templates for emails

These are the templates that had to be
within the last commit.

commentaries/templates/commentaries/vet_commentary_email_accepted.html

+Dear {{commentary.requested_by.get_title}} {{commentary.requested_by.user.last_name}},
+
+The Commentary Page you have requested, concerning publication with title {{commentary.pub_title}} by {{commentary.author_list}}, has been activated at https://scipost.org/commentary/'{{commentary.arxiv_or_DOI_string}}.
+You are now welcome to submit your comments.
+
+Thank you for your contribution,
+The SciPost Team.

commentaries/templates/commentaries/vet_commentary_email_modified.html

+Dear {{commentary.requested_by.get_title}} {{commentary.requested_by.user.last_name}},
+
+The Commentary Page you have requested, concerning publication with title {{commentary.pub_title}} by {{commentary.author_list}}, has been activated (with slight modifications to your submitted details).
+You are now welcome to submit your comments.
+
+Thank you for your contribution,
+The SciPost Team.

commentaries/templates/commentaries/vet_commentary_email_rejected.html

+Dear {{commentary.requested_by.get_title}} {{commentary.requested_by.user.last_name}},
+
+The Commentary Page you have requested, concerning publication with title {{commentary.pub_title}} by {{commentary.author_list}}, has not been activated for the following reason: {{refusal_reason}}.
+
+{% if further_explanation %}
+Further explanations:
+{{further_explanation}}
+{% endif %}
+
+Thank you for your interest,
+The SciPost Team.