Add copyright/attribution message to downloaded map images

[mmd-osm]

This PR adds OSM attribution to the map export, both for vector and raster formats. Besides, this PR adds webp image format.

I introduced a new dependency on python3-pil package to enable the png->{webp,jpeg} conversion.

Fixes openstreetmap/openstreetmap-website#551

[mmd-osm]

So regarding the temporary file vs. BytesIO topic. I looked at both options and thought it might be safer to store intermediate results on disk, rather than trying to juggle everything in memory. I also noticed the "resource.setrlimit" at the beginning and thought that memory usage is something to watch out for.

Are temporary files too much of a concern?

[tomhughes]

The memory limit is mostly just a backstop against people doing stupid things, especially with the vector output formats as I recall.

Temporary files generally make me nervous because you need to be sure they will always get cleaned up properly, especially if like here you're not immediately deleting them, which you can't if you need to be able to pass them to functions by name. Though all the functions here could actually take a handle instead I think.

[pnorman]

memory usage is something to watch out for.

I suspect this is because Mapnik can get memory crazy with some output formats. The memory limit is 4GB, so I don't see that storing the rasterized output in-memory is an issue.

PDF and PS are terrible to use with Mapnik and likely to break stuff in other ways, so I wouldn't worry about those.