Icons migrated from alphagov static

Icons taken from https://github.com/alphagov/static

There is some inconsistency in the way icons/images are stored in govuk_frontend_toolkit. Some are prefixed with ```icon-``` while others are prefixed with with the context e.g. ```player-icon-```. There is also a subdirectory for icon-steps, but not for the player icons. I did consider an ```icon-general``` sub directory for the new images, open to doing that if you think it appropriate.

(This directory may need a tidy up, but ensuring backwards compatibility might be a bit tricky.)