This adds `text_for`, `govspeak_for` and `html_for` methods intended
for the smart answer flow methods as a way for users to define content
for a smart answer. These are intended to replace the
`render_content_for` helper that has logic to determine the content
type.

This is set-up to be backwards compatible with the render_content_for
method so that both can co-exist for a period of time to allow
migration.

In doing this I also simplified a few things. We don't make any use of
options when calling `content_for` so these methods don't need to
support an option. All our usages of these methods use a block to add
content so I've set it up to raise an error.

I also removed the need to remove excess new lines after govspeak is
rendered. Since we already have HTML at that point I'm not sure what
value that adds.

This change allows template files to lack the .govspeak part of their
filename. For example lib/smart_answer_flows/business-coronavirus-support-finder/business_coronavirus_support_finder.govspeak.erb
can be renamed lib/smart_answer_flows/business-coronavirus-support-finder/business_coronavirus_support_finder.erb

This is done to reflect that these files no longer only contain govspeak
(arguably they never did since they were a mix of govspeak and plain
text) and thus the govspeak extension adds confusion. Since they are
mixed media it makes more sense for them to just have a ".erb"
extension.

Rendering and looking up of templates are changed to be done by the
template lookup system of action view rather than the previous manual
path manipulation. The lookup work only seems to be used as a means to
output extra help text in views, which seems potentially unnecessary. I
didn't want to remove this functionality here though as I don't know if
someone does use it.

I did move the erb_template_path to be private since nothing external
seems to use it.

These tests were all tested functionality defined outside this test.

This adds in a check when specifying HTML in a smart answer (via
govspeak_for or html_for) that it is not for a field that is text only.
This is put in to address a concern that it could be rather easy for
someone to specify `govspeak_for(:title) { "my govspeak" }` and this
produce an unexpected output that no-one notices.

As part of this I removed the govspeak option in DEFAULT_FORMATS as I
didn't think it affected the outcome of any method calls.

This updates the documentation to remove references to .govspeak.erb
files and to make suggestions of which helper method (text_for,
govspeak_for, html_for) in each scenario.

Along the way I noticed that the label field may not be working (issue
prior to these changes) so I left a note about that so the next person
who wants it can investigate.

I removed the information on error_* as I believe that is mostly set on
a custom basis so doesn't need to be included in the general
documentation.

This uses the new text_for & govspeak_for helpers and drops the
.govspeak extension from the filename.