Don't persist map style attribution in URL

[ebrelsford]

This is an issue with, for example, this URL:

https://stamen.github.io/maperture/#map=16.78/40.8579/-73.9262/0/0&showCollisions=false&showBoundaries=false&showDiff=false&viewMode=swipe&maps=%5B%7B%22id%22%3A%22stamen-toner%22%2C%22name%22%3A%22Stamen%20Toner%22%2C%22type%22%3A%22leaflet%22%2C%22url%22%3A%22https%3A%2F%2Fstamen-tiles.a.ssl.fastly.net%2Ftoner%2F%7Bz%7D%2F%7Bx%7D%2F%7By%7D.png%22%2C%22attribution%22%3A%22Map%20tiles%20by%20%3Ca%20href%3D%5C%22http%3A%2F%2Fstamen.com%5C%22%3EStamen%20Design%3C%2Fa%3E%2C%20under%20%3Ca%20href%3D

Adding the attribution to the URL isn't particularly helpful and makes the URL unnecessarily gigantic--let's remove it.