Add ability to override service name on a page

[joelanman]

This is to fix #129

The fix is to not use include for the 'propositional header' but have the code directly in layout.html. In nunjucks, you can't define a block in an include from outside that include.

We don't really need to use include in this situation, as the header is not used elsewhere.

The example page looks like this:

[joelanman]

relevant section from nunjucks docs:

An included template does not participate in the block structure of its including template; it has a totally separate inheritance tree and block namespace. In other words, an include is not a pre-processor that pulls the included template code into the including template before rendering; instead, it fires off a separate render of the included template, and the results of that render are included.

[colinrotherham]

All works fine for me.

I was a bit confused (at first) seeing serviceName set in both config.js and service_name available as a content block in override-service-name.html—i.e. “which one do I edit??”.

But it all makes sense now.

(This might break naming convention but perhaps "blocks within blocks" should follow a service_name_override pattern?)