General optimisations to SVG files

Most of these are removals of unneeded bits of information that don’t actually do anything.

`xml:space="preserve"` doesn’t do anything unless you have descendant nodes that consist only of whitespace, but it’s added in by editors for unknown reasons.

svg x and y attributes set to 0px won’t do anything  because the default is 0. Added in by editors for no reason again.

`enable-background` this is added by editors even though it also [doesn’t do anything][what does enable-background do?]. As I understand it, it’s actually been deprecated because implementation isn’t possible, and only IE10 ever even implemented it. I don’t know the exact details but no one is really going to get any benefit from including it.

`xmlns:xlink` is usually added in and defined by editors, but then that namespace isn’t used within the SVG, so I’m not sure what they’re thinking there either.

The svg DOCTYPE declaration is also added in by editors for no reason. It actually seems to be [recommended not to use it][do not use svg doctype].

I don’t know what your preferred method of indentation is, whether it’s spaces or tabs, but the indentation wasn’t consistent across files so I fixed that and then changed them to spaces. That’s the very last commit for a reason though, so if you don’t agree, the change can be easily ignored.

[what does enable-background do?]: http://stackoverflow.com/questions/21354116
[do not use svg doctype]: https://jwatt.org/svg/authoring/