Add a help page that explains trial mode

> We start in trial mode and there are a bunch of things that we need to know, so
> let's explain this with a page, accessed from the footer. Not requiring log in.
> Should explain:
> 50 messages per day
> Can only send to yourself or team members
> How to go live
> We can then link to this from the dashboard (and any other place) where we
> tell you that you're in trial mode.

https://www.pivotaltracker.com/story/show/115775751

app/templates/admin_template.html

@@ -74,6 +74,7 @@
 
 {% block footer_support_links %}
   <nav class="footer-nav">
+    <a href="{{ url_for("main.help") }}">Help</a>
     <a href="https://docs.google.com/forms/d/1AL8U-xJX_HAFEiQiJszGQw0PcEaEUnYATSntEghNDGo/viewform">Support and feedback</a>
     <a href="{{ url_for("main.cookies") }}">Cookies</a>
     Built by the <a href="https://www.gov.uk/government/organisations/government-digital-service">Government Digital Service</a>