Don’t let users in trial mode send letters

Users in trial mode haven’t signed the MOU. This means that they haven’t
agreed to pay for any costs they incur.

Unlike text messages and emails, we don’t give you any free allowance of
letters. Sending _any_ letters will cost the user money.

Therefore we shouldn’t let users who haven’t agreed that they will pay
for the service to incur costs by sending letters.

The pattern used for this is roughly the same as other trial mode errors
that we have already, ie a red box that says you’re not allowed. Not
sure if this is exactly right because it’s not exactly an error so the
pattern might feel too heavy-handed.

Getting this in place means we can turn letters on for users in trial
mode without worrying that they’ll accidentally send real letters, which
would result in:
- us having to absorb those costs
- some awkward conversations

app/templates/views/check/column-errors.html

@@ -87,6 +87,17 @@
 
         {% include "partials/check/too-many-messages.html" %}
 
+      {% elif trying_to_send_letters_in_trial_mode %}
+
+        <h1 class='banner-title' data-module="track-error" data-error-type="Trying to send letters in trial mode" data-error-label="{{ upload_id }}">
+          You can’t send
+          {{ 'this letter' if count_of_recipients == 1 else 'these letters' }}
+        </h1>
+        <p>
+          In <a href="{{ url_for('.trial_mode') }}">trial mode</a> you
+          can only preview how your letters will look
+        </p>
+
       {% endif %}
 
       {{ skip_to_file_contents() }}