Store base64 data with linebreaks

[Chris Warburton]

We've got a bunch of images in data URIs, stored as base64. That's fine,
but it leads to very long lines, which is annoying when we're editing,
grepping, etc.

Many browsers will accept newlines in these URIs, but it seems to be
technically incorrect to put them in (which tidy-html will complain to
us about), so we shouldn't have them in our rendered result. We should
still put them in, splitting to 80 columns, for our Markdown source
though.

We can currently do this with calls out to PanPipe, using e.g.
`tr -d '\n'` to strip them out before insertion. This seems a bit
heavy-handed though, and a lot of boilerplate. Let's just have the
`render` script look for data URIs with base64 data, and strip newlines
from them.