feat: add polygon overlays to web ui

[cthulhustig]

Add support for a new 'polygon' overlay type.

Testing:

• Verified, if no line or fill attributes were specified, a filled polygon was drawn using colour specified by the style attribute
• Verified, if just the line attribute was specified, an outline polygon was drawn using the specified colour
• Verified, if just the fill attribute was specified, a filled polygon was drawn using the specified colour
• Verified, if the line and fill attribute were specified, the polygon was drawn filled and outlined in the respective colours
• Verified, if the w attribute was specified when drawn an outline polygon, the line width was set accordingly
• Verified, if the w attribute was not specified when drawn an outline polygon, then the line width defaulted to 1 pixel
• Regression tested that rect and circle overlays still displayed correctly when created with url queries

[cthulhustig]

This is another change I'd like to get merged so I can add some custom overlays to Auto-Jimmy. I've tried to keep how it's used consistent with the other overlay types but if there is anything you want me to change, just let me know. I'm not sure 'w' is the best choice of variable name for the line width, I only chose it as it was similar to how the radius is specified for the circle overlay.

[inexorabletash]

LG, merged & deployed!

[cthulhustig]

Thanks mate