Override gallery base url when prompting user

[ChrisBAshton]

As of Wraith v3.0.0, Wraith now prompts the user to open the gallery as follows:

View the gallery in your browser:
   file:///Users/ashtoc03/Sites/bbc/idt-product-tests/wraith/latest/news--stage/gallery.html

This is ok locally, but doesn't work very well in all situations, e.g. if you run Wraith with Docker:

file:///wraithy/latest/news--stage/gallery.html

...or on CI:

file:///jenkins/etc/etc/etc//news--stage/gallery.html

So it would be nice to add a property to the config, e.g.

gallery_base_url: "https://jenkins.mysite.co.uk/job/my-job/workspace/"

...which then gets outputted as:

View the gallery in your browser:
   https://jenkins.mysite.co.uk/job/my-job/workspace/idt-product-tests/wraith/latest/news--stage/gallery.html

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OR, I'd consider removing this prompt altogether. Not entirely sure how useful it is.

[jimpriest]

I'll second this request. Trying to hack something up in CI and it would be nice to define this URL ourselves.