Use render template: instead of render file:

We did a similar change in commit 47925fbab, and we were getting a
warning in Rails 6.0:

DEPRECATION WARNING: render file: should be given the absolute path to a
file

Since using `render file:` would ignore views in custom folders, we're
using `render template:` instead.

app/views/proposals/created.html.erb

@@ -27,4 +27,4 @@
   </div>
 </div>
 
-<%= render file: "proposals/show.html.erb", locals: { preview: true } %>
+<%= render template: "proposals/show", locals: { preview: true } %>