Consider switching default Express page type from `page_fillable` back to `page_fluid`

[wch]

This came up in a conversation with @jcheng5: I merged #850, which changed the default from page_fluid to page_fillable, but there was a drawback to page_fillable that didn't occur to me: if there are many plots or cards on a page, they might get compressed so much that they look bad, or, for plots, so that they don't work at all. The same can happen if there's a fill item (plots, cards, etc) along with content that doesn't automatically resize.

Here's a screenshot of what can happen:

[gshotwell]

We should probably do this and then maybe implement some vertical scrolling layout (page article or whatever) for express. Is it possible to make page_fillable only fill horizontally? That might work.

[jcheng5]

I think @cpsievert and I were thinking page_fixed instead (and I might argue for a narrower variant of page_fixed, the bootstrap version is so wide that it is all but indistinguishable from page_fluid most of the time IMHO)

[wch]

I'm fine with page_fixed.