Look up the asset host using Plek

Fall back to asking Plek for the asset host if the STATIC_DEV 
environment variable isn't set. This only applies in development and 
will result in the same behaviour as present, however it makes it easier 
to override the URL for a particular service across the whole stack 
using environment variables.
Don't blow up when Content API is unavailable

In Smart Answers, the Content API is only used to populate the related 
items, sections, and associated metadata. If an artefact doesn't exist, the app continues to operate without the metadata.

However, if the Content API is completely unavailable (eg. if you are 
running Smart Answers in isolation), Smart Answers blows up with a
GdsApi::HTTPErrorResponse.

This change catches that exception and returns an empty hash, so that 
the page can still be rendered without the artefact metadata.
Set a short expiry time when an artefact isn't present
Merge pull request #703 from alphagov/make-it-easier-to-run-on-its-own

Make Smart Answers easier to run on its own