Add TG icon

[IanMayo]

@hlaposhka - I now realise how the icons are created. It's a white SVG over a filled rectangle of the required color. The corners of the rectangle are rounded to give the circular icon.

You can see how it is done by inspecting an icon on this map: https://serge-review.herokuapp.com/storybook/?path=/story/local-mapping--default

So, I think we just need the "TG" letters in the right size, in white.

I've attached a sample SVG file with the correct (white) styling.

merchant-vessel.svg.zip

Thanks!

[hlaposhka]

@IanMayo I'm glad that we figured it out, I just remember how I created these icons and that initially, they went without a circle, this raised doubts)

I updated the color and deleted the extraversion with a circle

[IanMayo]

@hlaposhka - new icon working perfectly:

[hlaposhka]

@IanMayo - excellent!