SVG spritesheet cleanup

[bhousel]

Most of our SVG graphics have been switched over to symbols, which are more flexible because you can <use> them anywhere in the code, and you don't have to keep track of their exact positions in the spritesheet, and you can change their styling on the fly, etc.

A few SVG graphics still use the old-style background image trick, and it's mostly the graphics in the walkthrough and some of the modal and social buttons.

Tracking here that I should:

• switch the remaining graphics to symbols and change the parts of the code to <use> them
• which lets us split all the svg source images into separate files and do away with the brittle spritesheet and idraw file

[bhousel]

This was done for the poi- icons.. They're here now: https://github.com/bhousel/temaki

[bhousel]

This is now done! All the graphics in iD are assembled from individual SVG files into the main spritesheet at build time. No more iD-sprite.src.svg or idraw project file. Also the spritesheets are namespaced now, since we have several to juggle.

This means that the new procedure for adding a symbol or graphic to iD is:

• create your graphic and drop it into one of the folders under svg/iD-sprite/*
• npm run dist to rebuild

I've also added a README with some more info.