These files contain mixed media so a .govspeak.erb filename is a bit
misleading. A plain .erb extension is more representative of how these
files contain a mixture of plain text, govspeak and (potentially) HTML.

Command used:
```
find . -name '*.govspeak.erb' -exec sh -c 'mv "$1" "${1%.govspeak.erb}.erb"' _ {} \;
```

There are some files in here that contain only govspeak, these tend to
be partials (for example lib/smart_answer_flows/marriage-abroad/outcomes/_contact_local_authorities.erb),
I don't think it's worth specifying these as .govspeak.erb currently as
I don't think that offers advantages to outweigh the challenges that a
mixed bag of extensions may create. Maybe this is something useful if we
started adding a number of .html.erb files.

This is no longer necessary, these will load fine with just the basename. This
makes them more consistent with common Rails partial loading paradigms.

This replaces the lone use of render_content_for to allow HTML rendering
with a html_for method.

This uses govspeak_for instead.

This sets all of these scenarios of setting text content to use the
text_for helper method.