Allow warning_text helper to accept nested content (blocks)

[NickColley]

We can use the warning_text help with a plain sentence to output a warning:

https://tdt-documentation.london.cloudapps.digital/amend_project/content/#insert-a-warning

If we want to put more complicated markup into this helper it does not work, for example a link.

We should consider making this helper work with blocks, for example.

<%= warning_text do %>
    <p>Hello, world <a href="#">Link to something</a></p>
<% end %>