Allow platform admins to change daily message limit

One less thing we have to go into the database to do, and remember to
manually clear the cache for after.

app/main/views/service_settings.py

@@ -36,6 +36,7 @@ from app.main.forms import (
     EstimateUsageForm,
     FreeSMSAllowance,
     LinkOrganisationsForm,
+    MessageLimit,
     PreviewBranding,
     RenameServiceForm,
     SearchByNameForm,
@@ -938,6 +939,23 @@ def set_free_sms_allowance(service_id):
     )
 
 
+@main.route("/services/<uuid:service_id>/service-settings/set-message-limit", methods=['GET', 'POST'])
+@user_is_platform_admin
+def set_message_limit(service_id):
+
+    form = MessageLimit(message_limit=current_service.message_limit)
+
+    if form.validate_on_submit():
+        current_service.update(message_limit=form.message_limit.data)
+
+        return redirect(url_for('.service_settings', service_id=service_id))
+
+    return render_template(
+        'views/service-settings/set-message-limit.html',
+        form=form,
+    )
+
+
 @main.route("/services/<uuid:service_id>/service-settings/set-email-branding", methods=['GET', 'POST'])
 @user_is_platform_admin
 def service_set_email_branding(service_id):