Use one page template for each of:
- choosing a message template
- adding recipients
- checking an email message
- looking at a job
This commit consolidates the two templates into one, and adds logic to show
the SMS message pattern or the email message pattern depending on the type of
template.
It also gives email templates a bit more width, because the body and the from
address tend to be quite long.
This commit parameterises all methods in the send view so that they can send
either emails or SMS messages.
It works out what kind of message it is sending from the `template_type`
property of the template object.
This means that the `Template` util class needs to know about these properties,
which means that this commit depends on:
https://github.com/alphagov/notifications-utils/pull/2
This commit does _not_ add tests for sending emails. The existing tests for
sending SMS still pass, but actually sending emails is outside the scope of
this story.
